Explain why the clean dissociation between spoken and written language is described as the key distinguishing feature of pure word-dumbness.

This dissociation matters because it shows that the underlying problem is specific to the production of spoken output itself, rather than reflecting a more general disturbance of language or thought that would also be expected to affect writing. Because writing draws on many of the same underlying language processes as speech, in most speech and language disorders some degree of written impairment accompanies spoken impairment; the fact that writing remains essentially normal here points toward a more narrowly circumscribed disruption specific to spoken output, which is clinically useful both for localising the underlying lesion and for planning how the patient can best communicate during recovery.