When I say I can make you understand why this is so, I only mean that I can answer the first “why” that any one is likely to ask about it, and perhaps a “why” or two behind this. Then I must stop. This is all that is ever meant by those who say they can tell us why a thing is so and so. No one professes to be able to reach back to the last “why” that any one can ask, and to answer it. Fortunately for philosophers, people generally become fatigued after they have heard the answer to two or three “whys” and are glad enough to let the matter drop. If, however, any one will insist on pushing question behind question long enough, he will compel us to admit that we come to the end of our knowledge which is based ultimately upon ignorance. To get knowledge out of ignorance seems almost as hopeless a task as to get something out of any number of nothings, but this in practice is what we have to do and the less fuss we make over it the better. When, therefore, we say that we know “why” a thing is so and so, we mean that we know its immediate antecedents and connections, and find them familiar to us. I say that the immediate antecedent of, and the phenomenon most closely connected with, heredity is memory. I do not profess to show why anything can remember at all, I only maintain that whereas, to borrow an illustration from mathematics, life was formerly an equation of, say, 100 unknown quantities, it is now one of only, inasmuch as memory and heredity have been shown to be one and the same thing. Memory i Memory is a kind of way (or weight—whichever it should be) that the mind has got upon it, in virtue of which the sensation excited endures a little longer than the cause which excited it. There is thus induced a state of things in which mental images, and even physical sensations (if there can be such a thing as a physical sensation) exist by virtue of association, though the conditions which originally called them into existence no longer continue. This is as the echo continuing to reverberate after the sound has ceased. ii To be is to think and to be thinkable. To live is to continue thinking and to remember having done so. Memory is to mind as viscosity is to protoplasm, it gives a tenacity to thought—a kind of _pied à terre_ from which it can, and without which it could not, advance.