The central idea, which dominates Locke's work, is that all our knowledge is derived from experience. But this does not strike us so much as a thesis to be maintained as a conclusion arrived at after a vast amount of patient thought and inquiry. Have we any ideas independent of experience? or, as Locke phrases it, are there any Innate Principles in the mind? "It is an established opinion amongst some men that there are in the Understanding certain Innate Principles, some Primary Notions, κοιναὶ ἔννοιαι [Greek: koinai ennoiai], characters, as it were, stamped upon the mind of man, which the Soul receives in its very first being and brings into the world with it." This is the opinion which Locke examines and refutes in the first, or introductory, book of the _Essay_. It has often been objected that he mistakes and exaggerates the position which he is attacking. And so far as his distinguished predecessor, Descartes, is concerned (though to what extent Locke has him in mind, his habit of not referring to other authors by name prevents us from knowing), this is undoubtedly the case. For Descartes, though he frequently employs and accepts the expression "innate notions" or "innate ideas," concedes, as so many philosophers of the same school have done since, that this native knowledge is only implicit, and requires definite experiences to elicit it. Thus, in his notes on the Programme of Regius, he expressly compares these innate notions or ideas with the nobility which is characteristic of certain ancient stocks, or with diseases, such as gout or gravel, which are said to be "innate" in certain families, not "because the infants of those families suffer from these diseases in their mother's womb, but because they are born with a certain disposition or tendency to contract them." Here Descartes seems to have been on the very point of stumbling on the principle of heredity which, in the hands of recent physiologists and psychologists, has done so much towards reconciling rival theories on the nature and origin of knowledge and clearing up many of the difficulties which attach to this branch of speculation. It must be confessed, however, that in his better-known works he often employs unguarded and unexplained expressions which might easily suggest the crude form of the _à priori_ theory attacked by Locke. Still more is this the case with other authors, such as Lord Herbert of Cherbury and Dr. Ralph Cudworth, whose works were in general circulation at the time when Locke was composing his _Essay_. Lord Herbert, though indeed he acknowledges that "common notions" (the expression by which he designates _à priori_ principles) require an object to elicit them into consciousness, seems invariably to regard them as ready-made ideas implanted in the human mind from its very origin. They are given by an independent faculty, Natural Instinct, which is to be distinguished from Internal Sense, External