This chapter, on the Primary Qualities of Matter, controverts the opinion of Sir W. Hamilton, that extension, as consisting of co-existent partes extra partes, is immediately and necessarily apprehended by our consciousness.  It cites, as well as confirms, the copious proof given by Professor Bain (in his work on the Senses and the Intellect) that our conception of extension is derived from our muscular sensibility:  that our sensation of muscular motion impeded constitutes that of filled space:  that our conception of extension, as an aggregate of co-existent parts, arises from the sense of sight, which comprehends a great number of parts in a succession so rapid as to be confounded with simultaneity—­and which not only becomes the symbol of muscular and tactile succession, but even acquires such ascendancy as to supersede both of them in our consciousness.  Confirmation is here given to this important doctrine, not merely by observations from Mr Mill himself, but also from the very curious narrative, discovered and produced by Sir W. Hamilton, out of a work of the German philosopher, Platner.  Platner instituted a careful examination of a man born blind, and ascertained that this man did not conceive extension as an aggregate of simultaneous parts, but as a series of sensations experienced or to be experienced in succession—­(pp. 232, 233).  The case reported from Platner both corroborates the theory of Professor Bain, and receives its proper interpretation from that theory; while it is altogether adverse to the doctrine of Sir W. Hamilton—­as is also another case, which he cites from Maine de Biran:—­

’It gives a very favourable idea of Sir W. Hamilton’s sincerity and devotion to truth (remarks Mr Mill, p. 247), that he should have drawn from obscurity, and made generally known, two cases so unfavourable to his own opinions.’

We think this remark perfectly just; and we would point out besides, in appreciating Sir W. Hamilton’s merits, that his appetite for facts was useful to philosophy, as well as his appetite for speculation.  But the person whose usefulness to philosophy we prefer to bring into the foreground, is Platner himself.  He spent three weeks in patient examination of this blind man, and the tenor of his report proves that his sagacity in interpreting facts was equal to his patience in collecting them.  The rarity of all such careful and premeditated observation of the facts of mind, appears to us one main reason why (what Mr Mill calls) the psychological theory finds so little acceptance; and why those who maintain that what now seems a mental integer was once a multiplicity of separate mental fragments, can describe the antecedent steps of the change only as a latens processus, which the reader never fully understands, and often will not admit.  Every man’s mind is gradually