The doctrine of Ignorance, so essential to Ferrier's system, he found it hard to make clear to others:--'I am astonished at your not seeing the use, indeed the absolute necessity, of a _true_ doctrine of ignorance. This blindness of yours shows me what I may expect from the public; and how careful I must be, if I would go down at all, to render myself perfectly clear and explicit. Don't you see that a correct doctrine of ignorance is necessary for two reasons--_first_, on account of the _false_ doctrine of ignorance universally prevalent, one which has hitherto rendered, and must ever render, anything like a scientific ontology impossible; and, _secondly_, because this correct theory of ignorance follows inevitably from my doctrine of knowledge? This, which I consider a very strong recommendation, an indispensable condition of the theory of ignorance, is the very ground on which you object to it. Surely you would not have me establish a doctrine of ignorance which was not consistent with my doctrine of knowledge. Surely I am entitled to deduce all that is logically deducible from my principles. Your meaning I presume is that my doctrine of ignorance flows so manifestly from my doctrine of knowledge that it is unnecessary to develop and parade it. There I differ from you. It flows _inevitably_, but I cannot think that it flows obviously. Else why was it never hit upon until now?... Don't tell me, then, that _my_ conclusions that matter _per se_, _Ding an sich_, is what it is impossible for us to be ignorant of, just _because_ it is absolutely unknowable (and for no other reason). Don't tell me that this conclusion is so obvious as not to require to be put down in black and white, when we find Kant and _every_ other philosopher drawing, but most erroneously, the directly opposite conclusion from the same premises. Matter _per se_, _Ding an sich_, was of all things that of which we were most ignorant!! and the ruin of metaphysics was the consequence of their infatuated blindness. Your objection, then, to my doctrine of ignorance, viz., that it is fixed in the very fixing of the doctrine of knowledge, and therefore does not require explication or elucidation, I cannot regard as a good objection. It is true that the one of these fixes the other; but it requires some amount of explanation and demonstration to make this palpable to the understandings even of the most acute, and I am not sure that even you (yes, put on your best pair of spectacles, you will need them) yet see how impossible it is for us to be ignorant of matter _per se_, or of anything which is absolutely unknowable.'