The admissions that pleasures differ in kind, and that actions are already classified; the acknowledgment that happiness includes the happiness of others, as well as of ourselves; the confusion (not made by Aristotle) between conscious and unconscious happiness, or between happiness the energy and happiness the result of the energy, introduce uncertainty and inconsistency into the whole enquiry. We reason readily and cheerfully from a greatest happiness principle. But we find that utilitarians do not agree among themselves about the meaning of the word. Still less can they impart to others a common conception or conviction of the nature of happiness. The meaning of the word is always insensibly slipping away from us, into pleasure, out of pleasure, now appearing as the motive, now as the test of actions, and sometimes varying in successive sentences. And as in a mathematical demonstration an error in the original number disturbs the whole calculation which follows, this fundamental uncertainty about the word vitiates all the applications of it. Must we not admit that a notion so uncertain in meaning, so void of content, so at variance with common language and opinion, does not comply adequately with either of our two requirements? It can neither strike the imaginative faculty, nor give an explanation of phenomena which is in accordance with our individual experience. It is indefinite; it supplies only a partial account of human actions: it is one among many theories of philosophers. It may be compared with other notions, such as the chief good of Plato, which may be best expressed to us under the form of a harmony, or with Kant's obedience to law, which may be summed up under the word 'duty,' or with the Stoical 'Follow nature,' and seems to have no advantage over them. All of these present a certain aspect of moral truth. None of them are, or indeed profess to be, the only principle of morals.