Aristotle's dictum that personal intercourse is essential to the continuance of friendship, as if they had lived together all their lives. In his second letter to Molyneux, dated Sept. 20, 1692, Locke says:--"You must expect to have me live with you hereafter, with all the liberty and assurance of a settled friendship. For meeting with but few men in the world whose acquaintance I find much reason to covet, I make more than ordinary haste into the familiarity of a rational inquirer after and lover of truth, whenever I can light on any such. There are beauties of the mind as well as of the body, that take and prevail at first sight; and, wherever I have met with this, I have readily surrendered myself, and have never yet been deceived in my expectation." Molyneux had thought of coming over to England on a visit to Locke in the summer of 1694. Locke, in a letter written in the following spring, after deprecating the risks to which his journey might expose him adds:--"And yet, if I may confess my secret thoughts, there is not anything which I would not give that some other unavoidable occasion would draw you into England. A rational, free-minded man, tied to nothing but truth, is so rare a thing that I almost worship such a friend; but, when friendship is joined to it, and these are brought into a free conversation, where they meet and can be together, what is there can have equal charms? I cannot but exceedingly wish for that happy day when I may see a man I have so often longed to have in my embraces.... You cannot think how often I regret the distance that is between us; I envy Dublin for what I every day want in London." In a subsequent letter, written in 1695, he writes:--"I cannot complain that I have not my share of friends of all ranks, and such whose interest, assistance, affection, and opinions too, in fit cases, I can rely on. But methinks, for all this, there is one place vacant that I know nobody would so well fill as yourself; I want one near me to talk freely with "de quolibet ente," to propose to the extravagancies that rise in my mind; one with whom I would debate several doubts and questions to see what was in them." Thomas Molyneux, the brother of William, a physician practising in Dublin, had met Locke during his stay in Holland. They shared a common admiration for Sydenham, and the correspondence with William Molyneux revived their friendship, though it never attained to nearly the same proportions as that between Locke and the other brother. A passage on what may be called the Logic of Medicine, in one of Locke's letters to Thomas Molyneux, is worth quoting:--"What we know of the works of nature, especially in the constitution of health and the operations of our own bodies, is only by the sensible effects, but not by any certainty we can have of the tools she uses or the ways she walks by. So that there is nothing left for a physician to do but to observe well, and so,