The sequence of the principles corresponds to four increasingly complex levels of knowledge. In intuition we are confronted simply with spatio-temporal magnitudes which make no explicit claim to objective reality, in perception we now encounter sensory contents which furnish substantive reality, which are then explicitly connected with one another in experience, and which, at the final level of empirical thought, are completed in the form of cognition proper. Within the third level of knowledge experiences in the plural sense are limited to specific areas of nature, such as light, heat or sound, while "experience" in the singular sense that is so important to Kant embraces nature in its entirety. Both senses together constitute the concept of experience in the narrower sense, experience as one of the four epistemic levels, as distinct from the broader concept of experience which, as the sum of all knowledge (B 296), includes all four levels. Experience in this sense is directed to the world in the broad sense, the 'sum of all appearances", which for Kant also includes the world in the narrower sense, the "mathematical whole" of all appearances, as well as nature, understood as the world 'so far as it is considered as a dynamical whole" (B 446). Thus the mathematical principles are concerned with the world in the narrow sense, while the dynamic principles are concerned with nature.