The Donnellan Lecturer for 1897-8 [1] took for his subject the doctrine of the Blessed Trinity in relation to contemporary idealistic philosophy. The scope of these lectures is, not to prove the doctrine of the Trinity philosophically, but to show that the difficulty besetting the conception of a multiplicity of persons united by a superpersonal bond, is just the same difficulty that brings idealistic philosophy to a dead-lock when it endeavours (1) to escape from solipsism, (2) to vindicate free-will,(3) to solve the problem of evil. He naturally speaks of Idealism as "the only philosophy which can now be truly called living," in the sense in which a language is said to live; that is, which is growing and changing, and endeavouring to bring new tracts of experience under its synthesis; which is current in universities of the day. Of the Realism which survives in the seminaries of the ecclesiastical world he naturally knows nothing; addressing himself to a wholly different public, he speaks to it on its own assumptions, in its own mental language; and indeed he knows no other. But having weighed idealism in the balance of criticism, he finds it far short of its pretensions to be an adequate accounting for the data of experience; he finds that it leads the mind in all directions to impassable chasms which only faith can overleap. It does not demand or suggest the mystery of the Trinity, but reveals a void which, as a fact that doctrine alone does fill. The convinced Realist will not be very interested about the problem of solipsism which for him is non-existent, but the proposed relief from the difficulties of free-will and of the existence of evil may be grateful to all indifferently; or at least may suggest principles adaptable to other systems. In his Trinitarian theology Mr. D'Arcy is in many points at variance with the later conclusions of the schools; and in some instances his argument depends vitally on this variance; but not in the main. For his main point is that as our own personality--the highest unity of which we have experience--takes under itself unities of a lower grade; so the doctrine of the Trinity implies what the hiatuses of philosophy require, namely, that personal unity is not the highest; that, beyond any power of our present conception, the personally many can be really (not only morally or socially) _one thing_. "A wonderfully unspeakable thing it is," says Augustine, "and unspeakably wonderful that whereas this image of the Trinity" _(sc.,_ the human soul), "is one person, and the sovereign Trinity itself, three persons, yet that Trinity of three persons is more inseparable than this trinity" (memory, understanding, and will) "of one person." This "superpersonal" unity is of course a matter of faith and not of philosophy, yet it is a faith without which subjective philosophy must come to a stand-still; it is as much a postulate of the speculative reason as God and immortality are of the practical reason.