An Essay Concerning Human Understanding - Book IV Chapter XIII: Some Further Considerations Concerning our Knowledge
1. Our knowledge partly necessary, partly voluntary. Our knowledge, as in other things, so in this, has so great a conformity with our sight, that it is neither wholly necessary, nor wholly voluntary. If our knowledge were altogether necessary, all men’s knowledge would not only be alike, but every man would know all that is knowable ; and if it were wholly voluntary, some men so little regard or value it that they would have extreme little, or none at all. Men that have senses cannot choose but receive some ideas by them ; and if they have memory, they cannot but retain some of them ; and if they have memory, they cannot but retain some of them ; and if they have any distinguishing faculty, cannot but perceive the agreement or disagreement of some of them one with another ; as he that has eyes, if he will open them by day, cannot but see some objects and perceive a difference in them. But though a man with his eyes open in the light, cannot but see, yet there be certain objects which he may choose whether he will turn his eyes to ; there may be in his reach a book containing pictures and discourses, capable to delight or instruct him, which yet he may never have the will to open, never take the pains to look into.
2. The application of our faculties voluntary ; but, they being employed, we know as things are, not as we please. There is also another thing in a man’s power, and that is, though he turns his eyes sometimes towards an object, yet he may choose whether he will curiously survey it, and with an intent application endeavour to observe accurately all that is visible in it. But yet, what he does see, he cannot see otherwise than he does. It depends not on his will to see that black which appears yellow ; nor to persuade himself that what actually scalds him, feels cold. The earth will not appear painted with flowers, nor the fields covered with verdure, whenever he has a mind to it : in the cold winter, he cannot help seeing it white and hoary, if he will look abroad. Just thus is it with our understanding : all that is voluntary in our knowledge is the employing or withholding any of our faculties from this or that sort of objects, and a more or less accurate survey of them : but, they being employed, our will hath no power to determine the knowledge of the mind one way or another ; that is done only by the objects themselves, as far as they are clearly discovered. And therefore, as far as men’s senses are conversant about external objects, the mind cannot but receive those ideas which are presented by them, and be informed of the existence of things without : and so far as men’s thoughts converse with their own determined ideas, they cannot but in some measure observe the agreement or disagreement that is to be found amongst some of them, which is so far knowledge : and if they have names for those ideas which they have thus considered, they must needs be assured of the truth of those propositions which express that agreement or disagreement they perceive in them, and be undoubtedly convinced of those truths. For what a man sees, he cannot but see ; and what he perceives, he cannot but know that he perceives.
3. Instance in numbers. Thus he that has got the ideas of numbers, and hath taken the pains to compare one, two, and three, to six, cannot choose but know that they are equal : he that hath got the idea of a triangle, and found the ways to measure its angles and their magnitudes, is certain that its three angles are equal to two right ones ; and can as little doubt of that, as of this truth, that it is impossible for the same thing to be, and not to be.
4. Instance in natural religion. He also that hath the idea of an intelligent, but frail and weak being, made by and depending on another, who is eternal, omnipotent, perfectly wise and good, will as certainly know that man is to honour, fear, and obey God, as that the sun shines when he sees it. For if he hath but the ideas of two such beings in his mind, and will turn his thoughts that way, and consider them, he will as certainly find that the inferior, finite, and dependent is under an obligation to obey the supreme and infinite, as he is certain to find that three, four, and seven are less than fifteen ; if he will consider and compute those numbers : nor can he be surer in a clear morning that the sun is risen ; if he will but open his eyes and turn them that way. But yet these truths, being ever so certain, ever so clear, he may be ignorant of either, or all of them, who will never take the pains to employ his faculties, as he should, to inform himself about them.