The main references in Descartes, outside the Meditations, to the notion of ‘clear and distinct’ perception  are contained in Discourse on Method — the first rule —  Regulae IlI and XII, the Principles of Philosophy and Replies to Objections — Appendix.

In Descartes’s “Principles of philosophy,” perception is said to be clear when it is "present and apparent to an attentive mind." 

We may compare with Descartes’s Regulae, where intuition is described as an "indubitable conception formed by an unclouded and attentive mind" — "unclouded" connects with "distinct".

In Descartes’s “Principles,” a perception is said to be *distinct* if it is so precise and different from all other objects that it contains within itself nothing but what is clear.

In the case of severe pain, perception is said to be very clear but may not be distinct, if it is confused with an *obscure* judgment about the cause of the pain — physical damage.

So a perception may be clear — without being distinct.

But a perception cannot be distinct — without being clear.

Descartes's account is obscured by  predominance of visual analogy in exposition, failure to distinguish between perception POTCH (conception) COTCH — of an thing, or object, or a concept, and perception (knowledge, assurance) of a proposition.

A Proposition might be Descartes’s primary concern.

And it is not too difficult to give a more or less precise interpretation to ‘clear and distinct’ perception of a proposition. 

A proposition is clearly and distinctly perceived by Descartes if Descartes has no doubt at all that it is true after having adequately — and perhaps successfully — satisfied himself just what it entails and does not entail — it is clear to Descartes what it contains, and so what it contains is clear to Descartes [clearly true], since Descartes cannot both have no doubt that p and have no doubt that p - > q, unless I have no doubt that q.

Descartes's failure to distinguish a thing, an object or a concept, on the one hand, and aproposition, on the other, comes out particularly clearly in Regulae.

Descartes’s account in Regulae of the knowledge of a “simple nature” and its “blending” or conjunction with another simple nature is fairly clearly an account of the supposed object of clear and distinct perception - or at least of a very important subclass of such objects.

A  "Simple nature" is, rightly, an unanalyzable concept of a high, but not too high, level of generality. 

“Figure” is a simple nature — entailing no more general con-cept.

Limit — i.e. “terminus” —  is not, though more general than figure, applying not only to a region of space but also to a stretche of time, for according to Descartes, the expression "limit" does not apply unambiguously to a space and to a time — presumably because of the *categorical* difference between a space and a time.

The Knowledge of a simple nature is said to be incapable of error — according to Descartes because of their simplicity —  but surely the cash value of this immunity is that a simple nature is not a propositions and so not the sort of things to be false — or true.

And Descartes speaks in one breath of knowledge of a simple nature — a concept — and of knowledge of their blending or combination (perhaps propositions) and of an "ax-iom" — certainly a proposition.

At the time of Regulae,  Descartes's position seems to have been: 

Certain knowledge is confined to intuition and deduction; 

intuition is infallible; 

deduction, which is a concatenation of intuitions, is fallible only insofar as memory-mistake may be involved 

— no account is taken of fallacies.

 Intuition, qua understanding/apprehension of a simple concept, is infallible, since I cannot misapprehend or fail completely to apprehend what has no internal complexity. 

The Intuition of a proposition is the recognition of a necessary connection between simple concepts; 

this consists in recognition of one concept as implicitly contained in another — cf. Leibniz, Wolff, and Kant on "analytic" versus “synthetic.”

So Descartes is in a position to hold that certain knowledge of a proposition is really only a matter of articulated understanding of concepts.

This is not at all absurd.

It resembles the view of an analytic proposition as one that cannot be denied by anyone who understands it — denial [conflicts with], counts against understanding.

All the same, for Descartes, it is incoherent.

A necessary connection between two simple concepts cannot consist in one concept's being implicitly contained in another, for the containing concept would have to be complex. 

It is not clear how far this line of thinking survives in Descartes's thought; 

"simple nature" is a technical term which does not appear in Descartes’s later work.

However, a possible confusion between the knowledge of a proposition and the understanding of a concept is detectable in the Meditations. 

Descartes's main use of “clear and distinct” perception is to provide a criterion of truth and certainty for a proposition.

But in the proof of the distinctness of mind and body, Descartes relies on the principle that 

if A can be clearly and distinctly conceived or understood apart from B, 

A and B are logically distinct and can exist separately. 

This can, of course, be represented as the “clear and distinct” perception (knowledge) of the modal proposition that 

it is possible that A should exist (be exemplified) when B does not exist.

But Grice suggests that Descartes thought of this proposition as grounded on the distinct conception of A (a conception not involving the conception of B).

Finally it is important to remember that although, for Descartes, the primary cases of “clear and distinct” perception is a necessary truth, not every case of “clear and distinct” perception is a necessary truth.

 "I exist"  — “Descartes exists.”

or

"I have a pain" 

is not a expression of a necessary truth, though Descartes — big ego that he had — may have failed to see clearly that the first is not.

One of Descartes’s Discourses - cf. Meditations — specifies the question at issue as being 

"what is requisite to the truth and certainty of a proposition?" 

and lays down the general rule that 

whatever we conceive COTCH — Medita-tions III "perceive" POTCH — very clearly and very distinctly is true," 

and adds that there is some difficulty in discerning which conception really is distinct.

One may wonder just why truth and certainty are spoken of so indifferently, since they are not identical notions — though there may be some inclination to suppose them to coincide in the area of *necessary* truth.

It is attractive — though because of Gödel, wrong — to equate mathematical proof with provability.

"Certain" occurs in at least two distinguishable contexts: 

"it is certain that p" 

label this "ob-jective" certainty)

"x is certain that p" 

label this "subjective"certainty.

Perhaps, then, Descartes is subscribing to two rules (conflated): 

whatever is clearly and distinctly perceived is objectively certain.

whatever is *objectively* certain is true.

It is fairly clear that Descartes wants to hold not only that if something is clearly and distinctly perceived, it is certain, but also that 

only if something is clearly and distinctly perceived is it certain 

— or at least that only if *we* are satisfied that *we* clearly and distinctly *perceive that* p, are we entitled to say that it is certain that p.

What status did Descartes attribute to his general rule? 

The natural supposition is that Descartes thought of it as itself a necessary truth. 

If it is a necessary truth, it might be either an implicit definition of "certain" or the specification of a sure sign or mark of the presence or certainty. 

But there are indications of a different interpretation. 

In one of his Discourses — first rule — Descartes speaks of accepting only 

"what presents itself to my mind so clearly and so distinctly that I had no occasion to doubt it." 

And in Replies  Descartes says:

"to begin with, directly we think that we rightly perceive something, we spontaneously persuade ourselves that it is true!" 

He goes on: 

"Further, if this conviction is so strong that we have no reason to doubt concerning that of the truth of which we have persuaded ourselves, there is nothing more to inquire about, we have here all the certainty that can reasonably be desired." 

This suggests two further possibilities of interpretation:

that the rule specifies a psychological fact about us 

— that

— we cannot but assent to what we clearly and distinctly perceive — or *think* that we clearly and distinctly perceive

Or 

that 

our only reasonable policy is to assent to what we think that we clearly and distinctly perceive.

There are altogether, then, four possible ways of viewing the rule.

Descartes’s rule might specify:

Necessary truth *defining* certainty

Necessary truth specifying sure sign of certainty

Psychological fact about when we have to give our assent

Only reasonable procedure for attribution of certainty

We shall revert to at least some of these.

Descartes regards the establishment of his general rule as consequential upon or derivative from his arrival at the certitude of his own existence. 

The Cogito.

But in what way? 

The step is obviously not supposedto be a deductive one, so of what kind is it?

It may be that Descartes thinks of it as an example of so-called "intuitive induction," being led to recognition of the general necessity of an A being a B by detecting the coinstantiation of A and B in a particular case. 

But whether or not Descartes believed this, the nature of such a step is extremely *obscure.*

Grice finds Descartes’s rambling on clear and distinct perception brutally obscure and confused.

It is not clear what function the individual case can have, other than to draw attention to the possibility of a general connection between A and B, to put the idea of a general truth into one's head. 

But the “Cogito; ergo, sum” does not seem especially qualified for this purpose, since the certainty of Descartes’s existence seems to depend not notably on clear and distinct perception, but rather on the fact that it is immune to the hypothesis of a malignant demon and the fact that "I exist" is one of a special class of propositions (statements)

— cf. "I am awake" — 

whose *truth* is required in order that their expression should count as the making of an assertion.

An utterance of 

“I exist" 

is either true or not a statement-making utterance at all. 

Descartes might just as well have had his attention drawn to the general rule by, for example, the simple arithmetical propositions which initially he seems to have regarded as open to doubt; 

indeed, if such examples were no good to him to begin with (as being questionable), they will remain questionable even after the general rule is accepted; and this Descartes does not want.

One may, of course, diagnose a condition A on which a particular feature B (e.g. certainty) depends by considering what is common to clear cases of B and seeing what we seem to go by in ascribing B; 

but for this we need consideration of a range of examples, not just a single one — e.g. the “Cogito; ergo, sum.”

And the existence of such a range — cf. mathematical examples — is just what Descartes seems initially to put in question.

The well-known Cartesian circle presents another difficulty.

Descartes seems to say that the acceptability of the general rule is dependent on the acceptability of the existence of a beneficent God— a malignant demon might deceive us even about what we clearly and distinctly perceive. 

But the existence of God needs proof, and the premises and conclusion of such a proof must be accepted on the grounds that they are clearly and distinctly perceived. 

But this involves a reliance on the criterion in advance of its guarantee from God.

In Replies, Descartes answers that he never intended the beneficence of God to guarantee the general rule.

What he intended it to guarantee is the reliance, in the conduct of a proof, on one's memory that certain propositions have been successfully proved, the proofs of which are not any longer before one's mind.

Descartes is being somewhat disingenuous in this answer.

Admittedly in Meditations Descartes does put forward just the position he outlines in Replies, but at the beginning of a different Meditation, Descartes does explicitly say that the use of the general rule to reestablish simple arithmetical propositions questioned in Meditations has to wait upon his proof of the existence of God. 

Descartes has in fact spoken with two voices, and will not admit it.

In any case, the favoured position is not without its own difficulties.

Two of Descartes's proofs of the existence of God are extremely elaborate and could not be conducted without an unguaranteed reliance on memory. 

However, AOSTA’s Ontological Proof is very short, and maybe Descartes could say that here reliance on memory is not involved.

It looks as if the beneficence of God will guarantee too much, for if it guarantees every reliance on memory, we should not be able to make the memory-mistakes we all know that we do make.

And if it guarantees only some memory, how do we characterize and identify the kind of memory that is guaranteed?

Descartes, it seems to Grice, has a perfectly good line at his disposal

here with regard to memory, analogous to the one he takes in Meditations about the material world. 

Very baldly put, Descartes’s position there is:

that 

There are all sorts of ordinary nophilosophical doubts and beliefs about the material world, which we are in a perfectly good position to resolve or correct, *provided* that we can rely on the general assumption that a sensory idea is generated by a material thing — and perhaps, it should be added, on the assumption of the legitimacy of certain checking procedures.

That a sensory idea is so generated — and perhaps that these procedures are legitimate — is a "lesson of nature":

something we are naturally disposed to believe.

But if a skeptical philosophical doubt is raised about it, we have no way of meeting such a doubt.

If our natural beliefs are incorrect, we have no way of discovering that they are. 

We need to know that God is no deceiver in order to be sure that we have not been constituted with a

[Grice is informed that Cartesian scholars no longer take seriously the suggestion that the function of Descartes's criterion is to justify a reliance on memory in the conduct of demonstration. 

This idea is, however, discernibly alive at the time when Grice’s essay was written.

built-in set of erroneous natural beliefs. 

For all this cf. Hume on a natural disposition.

Similarly, Descartes could say that we are in a position to correct or confirm an erroneous or dubious memory claim, an idea of memory, provided that we can assure that a memory claim is in general generated by a past event and a past situation, and provided that certain checking procedures — considerations of recency, distinctness, and coherence - are legitimate. 

But if the skeptic attacks these, we have no recourse, save to the beneficence of God, which would preclude our having been created with a natural tendency to assume that a memory idea in general corresponds with the past, when in fact there is no such general correspondence. 

The proof of God's existence is required solely to defend us from the Skeptic and does not provide for the infallibility of memory.

It has been argued by Prichard in "Knowledge and Perception" that Descartes is attempting to fulfill an impossible task, namely to provide a universally applicable mark or criterion of certainty (or, what comes to the same thing) of knowledge. 

Descartes is trying to specify a mark M — being a state of “clear and distinct” perception — such that, if and only if we can recognize our state of mind as regards some proposition p as exemplifying M can we call it a state of knowledge that p. 

Any such attempt fails on account of two different vicious circles/regresses. 

To know that a state S is a state of knowledge that p, we need to know that it exemplifies mark M; 

but to know this, we need to know that our state S, with regard to the proposition that S exemplifies the mark M itself exemplifies the mark M and so on. 

To know that the general rule is true, we have to know that our state of mind with respect to the general rule exemplifies the mark M.

But this information is no use to us, unless we can already use the rule — i. e., already know it to be true. 

We have to use the rule to certify itself.

These objections may well be fatal to any attempt to provide an absolutely general sure sign of certainty.

But Descartes may not be making such an attempt. 

The objections would not, Grice thinks,  apply against interpretation of the definitional variant.

But if we take Descartes in this way, there are other objections. 

For while it might be legitimate to define 

“Descartes is certain that p" 

as 

“Descartes clearly and distinctly perceives that p," 

it would not be so attractive to attempt to define 

It is certain that p" (objective certainty) in terms of clear and distinct perception. 

Indeed, the problem about certainty might be posed as the question when and how a step from 

“I am certain" 

to

"it is certain"

is justified.

Grice is inclined to think that Descartes is, not very clear-headedly, espousing interpretation (4), insofar as this is distinct from interpretation (3). 

To amplify this, Grice will mark some distinctions which may be of general philosophical interest. 

For any proposition, or range of propositions, three different kinds of conditions may be specifiable, which Grice shall call:

Conditions of Truth

Conditions of Establishment

Conditions of Re-Assurance

Let Grice consider these in relation to a class of propositions in which Descartes is specially interested, namely a mathematical proposi-tion.

Conditions of Truth will be explicit or implicit definitions. 

For any given proposition, or propositions, a wide variety of alternative specifications may be available.

Which one selects will depend on one's interests, on what the concepts are to which one is concerned to link the concepts involved in the original proposition or range of propo-sitions. 

One might, for certain purposes, wish to specify the truth-conditions for * = z: * = z is true iff the result of adding z to itself

y - 1 times is identical with x.

Establishment-conditions would be specifiable for a given system of mathematical propositions. 

A proposition p would be established if there has been found a proof of it within the system; if, that is, starting from such-and-such axioms, it has been possible to reach, in a finite number of steps constructed in accordance with such-and-such inferential rules, an expression of p.

The establishment-conditions will specify a procedure or achievement, which, if successfully realized, guarantees that p. 

But the question might arise whether the achievement or procedure has after all been successfully realized, whether something may not have gone wrong, and if such a question is not disposed of, we are not in a position to say "it is certain that p," even though in fact nothing may have gone wrong. 

So we need directions like "Go over the proof again (and if necessary again), looking out for misapplications of inferential rules, etc."

Such specifications of reassurance-conditions have two notable features: 

They are exceedingly unexciting, though supplementary directions, about what sort of mistakes to look out for, may be of general interest.

And just because the specifications are liable to be general in character and

unexciting, to put this into execution may require considerable skill and intelligence; it is not a mechanical operation. 

Reassurance-conditions are open-ended: 

there is no point at which carrying them out is finally completed.

One can always check again, though at some point (usually quite soon) it will become unreasonable to insist on further checking. 

But there is no general way of specifying precisely when that point is reached.

A partial application of these ideas to a proposition about a physical thing may have some philosophical point. 

What would be an appropriate method of specifying truth-conditions in a general form for a proposition about a material thing is not clear to Grice, and he shall not attempt the task. 

But it is fairly clear how establishment-conditions should be specified, at least for the optimal or favoured method of establishing a central class of propositions about a material thing — those about "medium-size" objects. 

To establish p in such cases is perceptually to observe that p. 

Since the achievement of perceptual observations may fail to be successfully realized — something may go wrong, not usually as the fault of the observer but rather as the fault of nature — we have re-assurance-directives such as "Make further observations, bring different senses into play, compare your observational findings with those of others etc." 

It seems to Grice that the phenomenalist may have made the mistake of taking what is a perfectly sound reassurance-directive and dressing it up so as to serve as a specification of truth-conditions for a proposition about a material thing in general. 

The stock objection to the phenomenalist, that his analysis is not completable and have to be supposed to be of infinite length, is worth bearing in mind here, for it may be a way of making the point that the open-endedness which is characteristic of reassurance-directives becomes objectionable if the attempt is made to convert reassurance-directives into an specification of a condition of truth.

The bearing of this discussion of Descartes is that Grice is suggesting that his general rule should be looked upon as an attempt to provide a reassurance-directive of maximal generality, one that will apply to any proposition which can ever be said to be certain, regardless of what their truth-conditions are, what their specific establishment-conditions are, and what more specific reassurance-directives are applicable once the establishment-conditions are identified. 

The directive is in effect:

 "Take all steps to satisfy yourself just what a given proposition entails and does not entail, and that having done this, you can find no ground for doubting the proposition in question."

Whether the provision of a maximally generalized reassurance-di-rective is a proper philosophical undertaking and whether, if it is, Descartes has adequately discharged it, are larger questions than it is the purpose of this essay to decide. 

Grice wishes to argue only that it is not obvious and has not been proved that it is an improper philosophical undertaking and that, if it is a proper undertaking, it is not easy to see how to improve upon Descartes's attempt to fulfill it. 

Our primary concern should, Grice thinks, be to ask, not whether Descartes's criterion is acceptable, but how and with what justification he has managed, by the application of what is apparently so unexceptionable a principle, to make at least plausible a skeptical position which is an affront to common sense.

 H. P. Grice.