Further meanings could also be enumerated but can be formulated by the reader for himself in the light of the ambiguities just noted.[221] The special context in each case can alone decide how the title is to be understood. If a really adequate definition of the purpose and scope of the _Critique_ is sought by the reader, he must construct it for himself. The following may perhaps serve. _The_ Critique _is an enquiry into the sources, conditions, scope and limits of our knowledge, both a priori and empirical, resulting in the construction of a new system of immanent metaphysics; in the light of the conclusions thus reached, it also yields an analysis and explanation of the transcendental illusion to which transcendent metaphysics, both as a natural disposition and as a professed science, is due._ Kant further complicates matters by offering a second division of the absolutely _a priori_,[222] viz. into the original and the derivative. Also, by implication, he classes relative _a priori_ judgments among the propositions to be reckoned with by the _Critique_; and yet in B 4 he speaks of the proposition, all bodies are heavy, as merely empirical.[223] =A criterion.=[224]--Necessity and universality are valid criteria of the _a priori_ (= the non-empirical). This follows from Kant’s view[225] of the empirical as synonymous with the contingent (_zufällig_). Experience gives only the actual; the _a priori_ alone yields that which cannot be otherwise. “Necessity and strict universality are thus safe criteria of _a priori_ knowledge, and are inseparable from one another. But since in the employment of these criteria the empirical limitation of judgments is sometimes more easily shown than their contingency, or since, as frequently happens, their unlimited universality can be more convincingly proved than their necessity, it is advisable to use the two criteria separately, each being by itself infallible.”[226]