In an open letter published in 1647, Descartes commented bitterly on the philosophical orthodoxy of his time:
The majority of those aspiring to be philosophers in the last few centuries have blindly
followed Aristotle. . . . And those who have not followed Aristotle . . . have nevertheless been
saturated with his opinions in their youth (since they are the only opinions taught in the Schools) and this has so dominated their outlook that they have been unable to arrive at knowledge of true principles ( AT IXB. 7; CSM I. 182).
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Since his early years, Descartes had seen it as his mission to replace the prevailing orthodoxy of his day with a new philosophical system. 2 The chief problem with the dominant
Aristotelianscholastic tradition, as Descartes saw it, was the weakness of its foundations or 'principles'. 'For every single problem ever solved by the principles distinctive of peripatetic [that is, Aristotelian] philosophy', Descartes wrote to Dinet in 1642, 'I can demonstrate that the supposed solution is invalid and false' ( AT VII.580; CSM II. 391). The term 'principle' had a more precise meaning in the seventeenth century than it does today, and was specifically used to refer to the starting points or fundamental axioms on which a philosophical or scientific system was based. 3 Descartes repeatedly complained that the 'schoolmen' had all been guilty of 'putting forward as principles things of which they did not possess perfect knowledge' ( AT IXB. 8; CSM I. 182).
It soon becomes clear that Descartes was not just alleging that the scholastic philosophers had got their principles wrong. His more serious complaint about the philosophy that
predominated in the schools was, in effect, that it lacked proper principles entirely--it failed to take seriously enough the need to push philosophical inquiry back to clear and self-evident startingpoints. An example which Descartes often cites in this connection is that of 'gravity'.
A commonly accepted explanation of why bodies fall was that they had the quality of gravitas or 'heaviness'; sometimes this was amplified by saying that it was 'of the nature' of terrestrial or earthly matter to find a place below that occupied by air (while, conversely, it was of the nature of airy particles to occupy a place above that of earthly). But, Descartes objected:
although experience shows us very clearly that the bodies we call 'heavy' descend towards the centre of the earth, we do not for all that have any knowledge of the nature of what is called 'gravity'--that is to say, the cause or principle which makes bodies descend in this way ( AT IXB. 8; CSM I. 182). 4
Descartes insisted that no concept should be allowed in a philosophical or scientific
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that are clear. Notions like 'gravity'--at least as used in scholastic physics--were, he believed, inherently vague. They might appear to correspond to something we experience every day, but if asked to analyse exactly what the supposed quality of 'heaviness' consisted in, the schoolmen were reduced either to silence, or to a barrage of jargon which would be no clearer (and often considerably more obscure) than what it was supposed to explain. 5 Complaints about the obscurity of scholastic jargon continued well after Descartes's death--indeed, as scholasticism came under increasing attack, the self-defensive jargon proliferated ever more vigorously. By the end of the century, we find Pierre Bayle, in his celebrated Dictionnaire historique et critique (first published 1697), commenting acidly on the 'public disputations where the Scholastics defend themselves with a jargon of distinctions that are suitable only for preventing the disappointment their relatives might have had in seeing them reduced to silence'. 6
The obscurities of the schoolmen were diagnosed by Descartes as resulting from their failure to appreciate that 'certainty does not lie in the senses, but solely in the understanding, when it possesses evident perceptions'. Instead of uncritical reliance on sensory experience, Descartes proposed an ideal of 'perfect knowledge' which he defined as knowledge which is 'deduced from first causes': 'if we are to set about acquiring perfect knowledge (and it is this activity to which the term "to philosophize" strictly refers) we must start with the search for first causes or principles' ( AT IXB. 2; CSM I. 179).
But how are we to discover such first causes or 'principles'? Descartes's striking claim--one that is reiterated many times throughout his writings--is that each of us possesses the innate power to uncover such principles, provided that our natural light of reason is directed aright. 7 This last proviso leads us to one of the best known features of Descartes's philosophy, and the feature that probably aroused the greatest interest in his own day, the idea of a distinctive method for reaching the truth. In his earliest major work, the Regulae, Descartes specifically addresses himself to this topic:
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By a 'method' I mean reliable rules which are easy to apply and such that if one follows them exactly one will never take what is false to be true . . . but gradually and constantly increase one's knowledge till one arrives at a true understanding of everything within one's capacity ( AT X. 372; CSM I. 16).
Crucial to Descartes's method is the notion of order. 'The whole method', he writes in Rule Five, 'consists entirely in the ordering and arranging of objects on which we must concentrate our mind's eye; we first reduce complicated and obscure propositions to simple ones, and then, starting with the intuition of the simplest ones of all, try to ascend through the same steps to knowledge of all the rest' ( AT X. 379; CSM I. 20). The model on which Descartes explicitly relies here is that of problem-solving in mathematics. The solution of difficult questions in arithmetic and geometry, he explains, involves just such a process of reducing the problem to its simplest essentials, and then proceeding in a step-by-step fashion from simple self-evident starting points to knowledge of ever more complex conclusions. 'Arithmetic and geometry are concerned with an object so pure and simple that they make no assumptions that experience might render uncertain, and they consist entirely in deducing conclusions by means of rational arguments' ( AT X. 365; CSM I. 12).
The simplicity of the starting-points in mathematics means, says Descartes, that the truths in question can be 'intuited'. This term can mislead the modern reader into supposing that some non-rational or non-cognitive faculty--a kind of 'hunch'--is involved. But Descartes's Latin term intueri carries, in its literal sense, the straightforward meaning of to 'look at' or 'look upon'; what Descartes is maintaining is that if an intellectual object (such as a triangle, say, or the number two) is sufficiently simple, then we can, with our mind's eye, just 'see' certain truths about that object (for example, that a triangle has three sides, or that two is one plus one) in a way that leaves no possible room for error:
By 'intuition' I do not mean the fluctuating testimony of the senses or the deceptive judgement of the imagination as it botches things together, but the conception of a clear and attentive mind which is so easy and distinct that there can be no room for doubt about what we are
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understanding. Alternatively, and this comes to the same thing, intuition is the indubitable conception of a clear and attentive mind which proceeds solely from the light of reason ( AT X. 368; CSM I. 14).
In his more mature writings, Descartes speaks, instead of intuition, of 'clear and distinct perception', but the basic comparison with ordinary ocular vision is preserved. 'I call a perception clear', he writes in the Principles of Philosophy, 'when it is present and accessible to the attentive mind--just as we say that we see something clearly when it is present to the eye's gaze and stimulates it with a sufficient degree of strength and accessibility' ( AT VIII. 22, CSM I. 207). A perception is distinct, Descartes goes on to explain, when, as well as being clear, it contains only what is clear. Descartes's point here is that although many of our perceptions have clear elements, they are often mixed up with elements that are not clear, so that the resulting judgement goes beyond what is directly present to the mind. Thus (to take Descartes's own example), if I report 'I have a pain in my leg', though the pain may present itself vividly enough to the mind, the judgement I make may none the less carry with it further implications which are not themselves clear: a certain kind of discomfort may be immediately present to my consciousness, but the implication that my leg is in a certain state goes beyond what I am immediately and directly aware of ( Principles I. 46). (Such further implications can, moreover, turn out to be false, as Descartes shows by his favourite example of the
'phantom limb' syndrome: he cites the case of a young patient who continued to complain that her hand was hurting even though the arm had in fact been amputated. 8 ) Just as with
'intuition', then, the crucial point about a 'clear and distinct perception' is the absolute simplicity of its content. A clear and distinct perception is selfevident because it is straightforwardly accessible to my 'mind's eye', and does not contain any extraneous implications which take me beyond that of which I am directly aware. Thus in the case of a simple mathematical proposition such as 'two and two make four', if I focus on the content of this proposition, I have right there, in front of my mind, all I need to be sure that the
proposition is true.
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