There are many evidences in these letters that Ferrier was not neglecting German Philosophy, for taking Experience as his basis he shows how it may be divided into _Wesen_ (_-an sich_), _Seyn_ (_für sich_), and the _Begriff_ (_anundfürsich_) on the lines of German metaphysics. As to the 'Common-Sense' Philosophy, he expresses himself in no measured terms: 'I am glad we agree in opinion as to the merits of the Common-Sense Philosophy. Considered in its details and accessories, it certainly contains many good things; but, viewed as a whole and _in essentialibus_, it is about the greatest humbug that ever was palmed off upon an unwary world. As an instance among many which might be adduced, of the ambiguity of the word, and of the vacillation of the members of this school, it may be remarked that while Reid made the essence of common-sense to consist in this, that its judgments are not conclusions obtained by ratiocination (_Works_, Sir W. Hamilton's edition, p. 425), Stewart, on the contrary, holds that these judgments are "the result of a train of reasoning so rapid as to escape notice" (_Elements_, vol. ii. p. 103). Sir W.'s _one hundred and six witnesses_ are a most conglomerate set, and a little cross-examination would try their mettle severely.' The most important part of Ferrier's system was his working out of the 'Theory of Ignorance,' in which, indeed, he might congratulate himself in having in great measure broken open new ground. He says of it: 'Hurrah, [Greek: eurêka], I have discovered the _Law of Ignorance_--and if I had a hecatomb of kain hens at my command I would sacrifice them _instanter_ to the propitious patron of metaphysics. Look you here. The Law of Knowledge is this, that, in order to know any _one_ thing we must always know two things; _hoc cum alio_--object plus subject--thing + me. This is the unit of knowledge. Analogously, only inversely, in order to be ignorant of any _one_ thing we must be ignorant of _two_ things--_hujus cum alio_--object plus subject--thing + me. This is the unit of ignorance.' Apparently, in spite of full explanation of his newly-discovered view, Ferrier's correspondent had failed to take it in, and consequently he gently rails at him for 'sticking at the axiom,' and wishes him to help him to a name for what he calls the 'Agnoiology' for want of something better. He goes on: 'I take it that I have caught you in my net, and that wallop about as you will I shall land you at last. I have now little fear that I shall succeed in convincing you, or at anyrate less hardened sinners, that the knowledge of object-subject is a self-contradiction, and that therefore object-subject, or matter _per se_, is not a thing of which we can with any sense or propriety be said to be ignorant. Be this as it may, you must at anyrate recognise in this doctrine a very great novelty in philosophy. The more incogitable a thing becomes, the more ignorant of it do _we_ become--that is the natural supposition.