We only know for sure his death date, it seems (neither Letham nor Muller give a birth-date for him). He died in 1173. He was a Scottish theologian. Both Muller and Letham indicate that he was a rationalist. However, he did have some helpful things to say on the Trinity. His influences include Augustine and Anselm (Letham, pg. 225).
Richard’s greatest Trinitarian dictum, to my mind, is this: “Love, by definition, is directed toward another. Therefore, love cannot exist where there is not a plurality of persons. Supreme love is not directed toward creation, since a created person is not worthy of supreme love” (Letham, pg. 225). He argues also that there cannot be merely two persons in the Godhead, otherwise love could not be shared (Letham, pg. 227). Letham’s assessment of this argumentation is as follows: Richard cannot prove why there shouldn’t be more than four persons in the Godhead. However, it is a very fine argument against a monistic God (such as the God Muslims have). See pg. 228.
Muller notes a further refinement to Richard’s argument, however, which might just answer Letham’s (small) criticism. Richard argues that the Father gives but does not receive (within the Godhead). The Son receives and gives, whereas the Spirit only receives. This is not to establish any kind of ontological inferiority, but only to establish what the lines of relationship look like within the Godhead. He argues that, with this setup, the Trinity is complete as three. “The only possibility remaining is a person who neither gives nor receives-but such a person is solitary, not a part of the common life of the three, so that a quaternity is excluded” (pg. 34). I think this more than adquately answers Letham’s criticism.
It is a great pity that Letham’s book was already in the final editing stage when Muller came out, thus precluding any chance of Letham interacting with Muller. However, the two balance each other out quite nicely. Letham is a bit thin on Reformation Trinitarianism (only deals in depth with Calvin). Muller fills in that gap nicely.