An Essay Concerning Human Understanding By John Locke Book IV: Knowledge and Probability

As to these general maxims, therefore, they are, as I have said, of great use in disputes, to stop the mouths of wranglers; but not of much use to the discovery of unknown truths, or to help the mind forwards in its search after knowledge. For who ever began to build his knowledge on this general proposition, WHAT IS, IS; or, IT IS IMPOSSIBLE FOR THE SAME THING TO BE AND NOT TO BE: and from either of these, as from a principle of science, deduced a system of useful knowledge? Wrong opinions often involving contradictions, one of these maxims, as a touchstone, may serve well to show whither they lead. But yet, however fit to lay open the absurdity or mistake of a man's reasoning or opinion, they are of very little use for enlightening the understanding: and it will not be found that the mind receives much help from them in its progress in knowledge; which would be neither less, nor less certain, were these two general propositions never thought on. It is true, as I have said, they sometimes serve in argumentation to stop a wrangler's mouth, by showing the absurdity of what he saith, [and by exposing him to the shame of contradicting what all the world knows, and he himself cannot but own to be true.] But it is one thing to show a man that he is in an error, and another to put him in possession of truth, and I would fain know what truths these two propositions are able to teach, and by their influence make us know which we did not know before, or could not know without them. Let us reason from them as well as we can, they are only about identical predications, and influence, if any at all, none but such. Each particular proposition concerning identity or diversity is as clearly and certainly known in itself, if attended to, as either of these general ones: [only these general ones, as serving in all cases, are therefore more inculcated and insisted on.] As to other less general maxims, many of them are no more than bare verbal propositions, and teach us nothing but the respect and import of names one to another. 'The whole is equal to all its parts:' what real truth, I beseech you, does it teach us? What more is contained in that maxim, than what the signification of the word TOTUM, or the WHOLE, does of itself import? And he that knows that the WORD whole stands for what is made up of all its parts, knows very little less than that the whole is equal to all its parts. And, upon the same ground, I think that this proposition, 'A hill is higher than a valley', and several the like, may also pass for maxims. But yet [masters of mathematics, when they would, as teachers of what they know, initiate others in that science do not] without reason place this and some other such maxims [at the entrance of their systems]; that their scholars, having in the beginning perfectly acquainted their thoughts with these propositions, made in such general terms, may be used to make such reflections, and have these more general propositions, as formed rules and sayings, ready to apply to all particular cases. Not that if they be equally weighed, they are more clear and evident than the particular instances they are brought to confirm; but that, being more familiar to the mind, the very naming them is enough to satisfy the understanding. But this, I say, is more from our custom of using them, and the establishment they have got in our minds by our often thinking of them, than from the different evidence of the things. But before custom has settled methods of thinking and reasoning in our minds, I am apt to imagine it is quite otherwise; and that the child, when a part of his apple is taken away, knows it better in that particular instance, than by this general proposition, 'The whole is equal to all its parts;' and that, if one of these have need to be confirmed to him by the other, the general has more need to be let into his mind by the particular, than the particular by the general. For in particulars our knowledge begins, and so spreads itself, by degrees, to generals [Footnote: This is the order in time of the conscious acquistion of knowledge that is human. The Essay might be regarded as a commentary on this one sentence. Our intellectual progress is from particulars and involuntary recipiency, through reactive doubt and criticism, into what is at last reasoned faith.]. Though afterwards the mind takes the quite contrary course, and having drawn its knowledge into as general propositions as it can, makes those familiar to its thoughts, and accustoms itself to have recourse to them, as to the standards of truth and falsehood. [Footnote: This is the philosophic attitude. Therein one consciously apprehends the intellectual necessities that were UNCONCIOUSLY PRESUPPOSED, its previous intellectual progress. In philosophy we 'draw our knowledge into as general propositions as it can' be made to assume, and thus either learn to see it as an organic while in a speculative unity, or learn that it cannot be so seen in a finite intelligence, and that even at the last it must remain 'broken' and mysterious in the human understanding. ] By which familiar use of them, as rules to measure the truth of other propositions, it comes in time to be thought, that more particular propositions have their truth and evidence from their conformity to these more general ones, which, in discourse and argumentation, are so frequently urged, and constantly admitted. And this I think to be the reason why, amongst so many self-evident propositions, the MOST GENERAL ONLY have had the title of MAXIMS.

12. Maxims, if care be not taken in the Use of Words, may prove Contradictions.

One thing further, I think, it may not be amiss to observe concerning these general maxims, That they are so far from improving or establishing our minds in true knowledge that if our notions be wrong, loose, or unsteady, and we resign up our thoughts to the sound of words, rather than [fix them on settled, determined] ideas of things; I say these general maxims will serve to confirm us in mistakes; and in such a way of use of words, which is most common, will serve to prove contradictions: v.g. he that with Descartes shall frame in his mind an idea of what he calls body to be nothing but extension, may easily demonstrate that there is no vacuum, i.e. no space void of body, by this maxim, WHAT IS, IS. For the idea to which he annexes the name body, being bare extension, his knowledge that space cannot be without body, is certain. For he knows his own idea of extension clearly and distinctly, and knows that it is what it is, and not another idea, though it be called by these three names, — extension, body, space. Which three words, standing for one and the same idea, may, no doubt, with the same evidence and certainty be affirmed one of another, as each of itself: and it is as certain, that, whilst I use them all to stand for one and the same idea, this predication is as true and identical in its signification, that 'space is body,' as this predication is true and identical, that 'body is body,' both in signification and sound.

13. Instance in Vacuum.

But if another should come and make to himself another idea, different from Descartes's, of the thing, which yet with Descartes he calls by the same name body, and make his idea, which he expresses by the word body, to be of a thing that hath both extension and solidity together; he will as easily demonstrate, that there may be a vacuum or space without a body, as Descartes demonstrated the contrary. Because the idea to which he gives the name space being barely the simple one of extension, and the idea to which he gives the name body being the complex idea of extension and resistibility or solidity, together in the same subject, these two ideas are not exactly one and the same, but in the understanding as distinct as the ideas of one and two, white and black, or as of CORPOREITY and HUMANITY, if I may use those barbarous terms: and therefore the predication of them in our minds, or in words standing for them, is not identical, but the negation of them one of another; [viz. this proposition: 'Extension or space is not body,' is] as true and evidently certain as this maxim, IT IS IMPOSSIBLE FOR THE SAME THING TO BE AND NOT TO BE, [can make any proposition.]

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