Defective is associated with defect, and deficient with deficit. (Fowler.) It follows that defective is appropriate primarily to what is wanting in quality, and deficient to what is wanting in quantity. That is a sound rule to follow, but it does not always give clear guidance, because it is not always clear whether what is wrong is a defect or a deficiency. Illumination may be bad because the lamps are poor in quality or because they are insufficient in quantity, and we may not know whether to call it deficient or defective. Someone who is "not all there" may be in that unfortunate state either because (as the expression suggests) his brains are insufficient in quantity or because they are below the standard quality. So we may call him indifferently either mentally defective or mentally deficient. But where the distinction is clear, in the interests of clarity of thought it should be preserved.
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