There is one important link in the deduction, as above given, which is hardly calculated to support the conclusions that depend upon it. Kant, as we have already noted,[905] asserts that the categories express the minimum of unity necessary for the possibility of apperception. A contention so essential to the argument calls for the most careful scrutiny and a meticulous exactitude of proof. As a matter of fact, such proof is not to be found in any part of the deductions, whether of the first or of the second editions. It is attempted only in the later sections on the _Principles of Understanding_, and even there it is developed, in any really satisfactory fashion, only in regard to the categories of causality and reciprocity.[906] This proof, however, as there given, is an argument which in originality, subtlety and force goes far to atone for all shortcomings. It completes the objective deduction by developing in masterly fashion (in spite of the diffuse and ill-arranged character of the text) the central contention for which the deduction stands. But in the transcendental deduction itself, we find only such an argument--if it may be called an argument--as follows from the identification of apperception with understanding. “The unity of apperception, in relation to the synthesis of imagination, is the understanding.... In understanding there are pure _a priori_ forms of knowledge which contain the necessary unity of pure synthesis of imagination in respect of all possible appearances. But these are the categories, _i.e._ pure concepts of understanding.”[907] The point is again merely assumed in A 125-6. So also in A 126: “Although through experience we learn many laws, these are only special determinations of still higher laws, of which the highest, under which all others stand, originate _a priori_ in the understanding itself....”[908] Again in A 129 it is argued that as we prescribe _a priori_ rules to which all experience must conform, those rules cannot be derived from experience, but must precede and condition it, and can do so only as originating from ourselves (_aus uns selbst_). ”[They] precede all knowledge of the object as [their] intellectual form, and constitute a formal _a priori_ knowledge of all objects in so far as they are thought (categories).”