The metaphysicians have spent many words in attempting to define the various qualities of the mind, and to account for a strong or a weak memory; but after all is said, we find that the surprising difference between different memories is unaccounted for; as unaccountable, indeed, as what differences the man of genius from the mere plodder. The principle of association of ideas is doubtless the leading element in a memory which is not merely verbal. We associate in our minds, almost instinctively, ideas of time, or space, or persons, or events, and these connect or compare one with another, so that what we want is called up or recalled in memory, by a train of endless suggestion. We all have this kind of memory, which may be termed the rational or ideal, as distinguished from the verbal and the local memory. The verbal memory is that which retains in the mind, and reproduces at will what has been said in our hearing by others, or what we have read which has made a marked impression upon us. Thus, some persons can repeat with almost exact accuracy, every word of a long conversation held with another. Others can repeat whole poems, or long passages in prose from favorite authors, after reading them over two or three times, and can retain them perfectly in memory for half a century or more. There have even been persons to whom one single reading of any production was sufficient to enable them to repeat it _verbatim_. These instances of a great verbal memory are by no means rare, although some of them appear almost incredible. John Locke tells us of the French philosopher Pascal, that he never forgot anything of what he had done, said, or thought, in any part of his natural life. And the same thing is recorded of that great scholar of Holland, Hugo Grotius. The mathematician Euler could repeat the Aeneid of Virgil from beginning to end, containing nearly nine thousand lines. Mozart, upon hearing the _Miserere_ of Allegri played in the Sistine Chapel at Rome, only once, went to his hotel, and wrote it all down from memory, note for note.