A most interesting effect of cortical disorder is _mental blindness_. This consists not so much in insensibility to optical impressions, as in _inability to understand them_. Psychologically it is interpretable as _loss of associations_ between optical sensations and what they signify; and any interruption of the paths between the optic centres and the centres for other ideas ought to bring it about. Thus, printed letters of the alphabet, or words, signify certain sounds and certain articulatory movements. If the connection between the articulating or auditory centres, on the one hand, and the visual centres on the other, be ruptured we ought _a priori_ to expect that the sight of words would fail to awaken the idea of their sound, or the movement for pronouncing them. We ought, in short, to have _alexia_, or inability to read: and this is just what we do have in many cases of extensive injury about the fronto-temporal regions, as a complication of _aphasic_ disease. Nothnagel suggests that whilst the _cuneus_ is the seat of optical _sensations_, the other parts of the occipital lobe may be the field of optical _memories and ideas_, from the loss of which mental blindness should ensue. In fact, all the medical authors speak of mental blindness as if it must consist in the loss of visual images from the memory. It seems to me, however, that this is a psychological misapprehension. A man whose power of visual imagination has decayed (no unusual phenomenon in its lighter grades) is not mentally blind in the least, for he recognizes perfectly all that he sees. On the other hand, he _may_ be mentally blind, with his optical imagination well preserved; as in the interesting case published by Wilbrand in 1887.[26] In the still more interesting case of mental blindness recently published by Lissauer,[27] though the patient made the most ludicrous mistakes, calling for instance a clothes-brush a pair of spectacles, an umbrella a plant with flowers, an apple a portrait of a lady, etc. etc., he seemed, according to the reporter, to have his mental images fairly well preserved. It is in fact the momentary loss of our _non_-optical images which makes us mentally blind, just as it is that of our _non_-auditory images which makes us mentally deaf. I am mentally deaf if, _hearing_ a bell, I can't recall how it _looks_; and mentally blind if, _seeing_ it, I can't recall its _sound or its name_. As a matter of fact, I should have to be not merely mentally blind, but stone-blind, if all my visual images were lost. For although I am blind to the right half of the field of view if my left occipital region is injured, and to the left half if my right region is injured, such hemianopsia does not deprive me of visual _images_, experience seeming to show that the unaffected hemisphere is always sufficient for production of these. To abolish them entirely I should have to be deprived of both occipital lobes, and that would deprive me not only