expect, to state the positive
meaning of _a priori_; but to give tests for what is _a priori_. Since
a test implies a distinction between itself and what is tested, it is
implied that the meaning of _a priori_ is already known.[5]

[5] It may be noted that in this passage (Introduction, Secs.
1 and 2) Kant is inconsistent in his use of the term 'pure'.
Pure knowledge is introduced as a species of _a priori_
knowledge: "_A priori_ knowledge, if nothing empirical is
mixed with it, is called pure". (B. 3, M. 2, 17.) And in
accordance with this, the proposition 'every change has a
cause' is said to be _a priori_ but impure, because the
conception of change can only be derived from experience. Yet
immediately afterwards, pure, being opposed in general to
empirical, can only mean _a priori_. Again, in the phrase
'pure _a priori_' (B. 4 fin., M. 3 med.), the context shows
that 'pure' adds nothing to '_a priori_', and the proposition
'every change must have a cause' is expressly given as an
instance of pure _a priori_ knowledge. The inconsistency of
this treatment of the causal rule is explained by the fact
that in the former passage he is thinking of the conception
of change as empirical, while in the latter he is thinking of
the judgement as not empirical. At bottom in this passage
'pure' simply means _a priori_.

The tests given are necessity and strict universality.[6] Since
judgements which are necessary and strictly universal cannot be based
on experience, their existence is said to indicate another source of
knowledge. And Kant gives as illustrations, (1) any proposition in
mathematics, and (2) the proposition 'Every change must have a cause'.

[6] In reality, these tests come to the same thing, for
necessity means the necessity of connexion between the
subject and predicate of a judgement, and since empirical
universality, to which strict universality is opposed, means
numerical universality, as illustrated by the proposition
'All bodies are heavy', the only meaning left for strict
universality is that of a universality reached not through an
enumeration of instances, but through the apprehension of a
necessity of connexion.

So far Kant has said nothing which determines the positive meaning of
_a priori_. A clue is, however, to be found in two subsequent phrases.
He says that we may content ourselves wi