The 'Observations on Man' (1749) is the first systematic effort to explain the phenomena of mind by the Law of Association. It contains also a philosophical hypothesis, that mental states are produced by the _vibration_ of infinitesimal particles of the nerves. This analogy, borrowed from the undulations of the hypothetical substance æther, has been censured as crude, and has been entirely superseded. But, although an imperfect analogy, it nevertheless kept constantly before the mind of Hartley the double aspect of all mental phenomena, thus preventing erroneous explanations, and often suggesting correct ones. In this respect, Aristotle and Hobbes are the only persons that can be named as equally fortunate. The ethical remarks contained in the 'Observations,' relate only to the second head of summary, the Psychology of Ethics. We shall take, first, the account of disinterestedness, and, next, of the moral sense. 1. _Disinterestedness_. Under the name _Sympathy_, Hartley includes four kinds of feelings:--(1) Rejoicing at the happiness of others--Sociality, Good-will, Generosity, Gratitude; (2) Grieving for the misery of others--Compassion, Mercy; (3) Rejoicing at the misery of others--Anger, Jealousy, Cruelty, Malice; and (4) Grieving for the happiness of others--Emulation, Envy. All these feelings may be shown to originate in association. We select as examples of Hartley's method, Benevolence and Compassion. Benevolence is the pleasing affection that prompts us to act for the benefit of others. It is not a primitive feeling; but grows out of such circumstances as the following. Almost all the pleasures, and few, in comparison, of the pains, of children, are caused by others; who are thus, in the course of time, regarded with pleasure, independently of their usefulness to us. Many of our pleasures are enjoyed along with, and are enhanced by, the presence of others. This tends to make us more sociable. Moreover, we are taught and required to put on the appearance of good-will, and to do kindly actions, and this may beget in us the proper feelings. Finally, we must take into account the praise and rewards of benevolence, together with the reciprocity of benefits that we may justly expect. All those elements may be so mixed and blended as to produce a feeling that shall teach us to do good to others without any expectation of reward, even that most refined recompense--the pleasure arising from a beneficent act. Thus Hartley conceives that he both proves the existence of disinterested feeling, and explains the manner of its developement.