This is that which I think great readers are apt to be mistaken in. Those who have read of every thing are thought to understand every thing too; but… - John Locke

Chapter VII
Of Simple Ideas of both Sensation and Reflection
1. Ideas of pleasure and pain.
There be other simple ideas which convey themselves into the mind by all the ways of sensation and reflection, viz. pleasure or delight,
and its opposite, pain, or uneasiness; power; existence; unity.

Nor need we fear that this philosophy, while it endeavors to limit our inquiries to common life, should ever undermine the reasonings of common life, and carry its doubts so far as to destroy all action, as well as speculation. Nature will always maintain her rights, and prevail in the end over any abstract reasoning whatsoever. Though we should conclude, for instance, as in the foregoing section, that, in all reasonings from experience, there is a step taken by the mind which is not supported by any argument or process of the understanding; there is no danger that these reasonings, on which almost all knowledge depends, will ever be affected by such a discovery. If the mind be not engaged by argument to make this step, it must be induced by some other principle of equal weight and authority; and that principle will preserve its influence as long as human nature remains the same. What that principle is may well be worth the pains of inquiry.

But yet possibly it will still be objected, suppose I wholly lose the memory of some parts of my life, beyond a possibility of retrieving them, so that perhaps I shall never be conscious of them again; yet am I not the same person, that did those actions, had those thoughts, that I once was conscious of, though I have now forgot them? To which I answer, that we must here take notice what the word I is applied to, which in this case is the man only. And the same man being presumed to be the same person, I is easily here supposed to stand also for the same person. But if it be possible for the same man to have distinct incommunicable consciousness at different times, it is past doubt the same man would at different times make different persons