The collection comprises seven letters, 1841 – 1846, between George Henry Lewes (1817 – 1878), English Philosopher and Critic of English Literature and Theatre, and James Frederick Ferrier (1808 – 1864), Scottish Metaphysical Philosopher, instigated by the appearance in October 1841 of an article by Ferrier in Blackwood’s Magazine entitled ‘ The Crisis of Modern Speculation’. The collection is an important and unique parcel of correspondence between the two Philosophers in which mutual respect is accompanied by serious differences of viewpoint on a crucial detail of Ferrier`s published argument. Challenged by Lewes on his expression concerning the relation between cause and effect in the process of human perception, Ferrier responds with detailed clarifications of his position that only serve to amplify Lewes` misgivings. The collection consists of Lewes` original letters and Ferrier`s original manuscript copies of his written responses. It also includes the original drafts and preliminary notes of Ferrier`s final unsent letter in which his confidence appears to falter.