The pathology of aphasia, as worked out by a combination of the experimental, the pathological and the anatomical lines of inquiry is a favourable example of what has been accomplished.
But, in spite of these materialistic tendencies, he followed Hume in reducing matter and everything knowable to phenomena of consciousness; and, supposing that nothing is knowable beyond phenomena, concluded that we can neither affirm nor deny that anything exists beyond, but ought to take up an attitude which the ancient sceptics called Aphasia, but he dubbed by the new name of Agnosticism.
'The view that this, with other mental processes, is a function of the brain, is remarkably corroborated by modern investigation of the disease of aphasia, where the power of thinking remains, but the power is lost of recalling the word corresponding to the thought, and this mental defect is found to accompany a diseased state of a particular locality of the brain (see Aphasia).
The particulars of his case have been investigated by Dr Bucknill and Sir William Wilde, who have proved that he suffered from nothing that could be called mental derangement until the "labyrinthine vertigo" from which he had suffered all his life, and which he erroneously attributed to a surfeit of fruit, produced paralysis, "a symptom of which was the not uncommon one of aphasia, or the automatic utterance of words ungoverned by intention.
Aphasia due to the local trouble and general decay then progressed rapidly together, and even then at 76, two more years were still to elapse before "he exchanged the sleep of idiocy for the sleep of death."