John Locke

“ All our knowledge consisting, as I have said, in the view the mind has of its own ideas, which is the utmost light and greatest certainty we, with our faculties, and in our way of knowledge, are capable of, it may not be amiss to consider a little the degrees of its evidence. The different clearness of our knowledge seems to me to lie in the different way of perception the mind has of the agreement or disagreement of any of its ideas. ”

“ Sensitive knowledge of the particular existence of finite beings without us. These two, viz. intuition and demonstration, are the degrees of our knowledge ; whatever comes short of one of these, with what assurance soever embraced, is but faith or opinion, but not knowledge, at least in all general truths. There is, indeed, another perception of the mind, employed about the particular existence of finite beings without us, which, going beyond bare probability, and yet not reaching perfectly to either of the foregoing degrees of certainty, passes under the name of knowledge. ”

“ Demonstration not limited to ideas of mathematical quantity. It has been generally taken for granted, that mathematics alone are capable of demonstrative certainty : but to have such an agreement or disagreement as may intuitively be perceived, being, as I imagine, not the privilege of the ideas of number, extension, and figure alone, it may possibly be the want of due method and application in us, and not of sufficient evidence in things, that demonstration has been thought to have so little to do in other parts of knowledge, and been scarce so much as aimed at by any but mathematicians. ”

“ There can be nothing more certain than that the idea we receive from an external object is in our minds : this is intuitive knowledge. But whether there be anything more than barely that idea in our minds ; whether we can thence certainly infer the existence of anything without us, which corresponds to that idea, is that whereof some men think there may be a question made ; because men may have such ideas in their minds, when no such thing exists, no such object affects their senses. ”

“ But ideas which, by reason of their obscurity or otherwise, are confused, cannot produce any clear or distinct knowledge ; because, as far as any ideas are confused, so far the mind cannot perceive clearly whether they agree or disagree. Or to express the same thing in a way less apt to be misunderstood : he that hath not determined ideas to the words he uses, cannot make propositions of them of whose truth he can be certain. ”

“ The secondary qualities of things not discovered by demonstration. Not knowing, therefore, what number of particles, nor what motion of them, is fit to produce any precise degree of whiteness, we cannot demonstrate the certain equality of any two degrees of whiteness ; because we have no certain standard to measure them by, nor means to distinguish every the least real difference, the only help we have being from our senses, which in this point fail us. ”

“ Knowledge not always clear, where the ideas that enter into it are clear. But since our knowledge is founded on and employed about our ideas only, will it not follow from thence that it is conformable to our ideas ; and that where our ideas are clear and distinct, or obscure and confused, our knowledge will be so too ? ”

“ Particles of light and simple ideas of colour. Whether then they be globules or no ; or whether they have a verticity about their own centres that produces the idea of whiteness in us ; this is certain, that the more particles of light are reflected from a body, fitted to give them that peculiar motion which produces the sensation of whiteness in us ; and possibly too, the quicker that peculiar motion is, – the whiter does the body appear from which the greatest number are reflected, as is evident in the same piece of paper put in the sunbeams, in the shade, and in a dark hole ”

“ I answer, That we certainly finding that pleasure or pain follows upon the application of certain objects to us, whose existence we perceive, or dream that we perceive, by our senses ; this certainty is as great as our happiness or misery, beyond which we have no concernment to know or to be. So that, I think, we may add to the two former sorts of knowledge this also, of the existence of particular external objects, by that perception and consciousness we have of the actual entrance of ideas from them, and allow these three degrees of knowledge, viz. ”

“ But this I think I may say, that I cannot (and I would be glad any one would make intelligible that he did) , conceive how bodies without us can any ways affect our senses, but by the immediate contact of the sensible bodies themselves, as in tasting and feeling, or the impulse of some sensible particles coming from them, as in seeing, hearing, and smelling ; by the different impulse of which parts, caused by their different size, figure, and motion, the variety of sensations is produced in us. ”

“ For, supposing the sensation or idea we name whiteness be produced in us by a certain number of globules, which, having a verticity about their own centres, strike upon the retina of the eye, with a certain degree of rotation, as well as progressive swiftness ”

“ That it is no great matter, whether I remove his scruple or no : where all is but dream, reasoning and arguments are of no use, truth and knowledge nothing. 2. That I believe he will allow a very manifest difference between dreaming of being in the fire, and being actually in it. ”