"Apprehension" (Auffassung) is a key term in Husserl's phenomenology of perceptual consciousness. However, its modes of operation have not yet been closely analyzed. Apprehension has its own principles and foundations. According to Husserl, the principles of apprehension are 1) contiguity, 2) equality and 3) similarity, and each of them expresses a specific kind of qualitative connection between the apprehension-content and the apprehension-sense. When a content presents a sense through equality or similarity, this sense can be regarded as a "projection" from the apprehension-content. Besides the principles, the working of perceptual apprehension as knowing is based on two foundations. The first foundation is a space of sense, in which a perceptual sense finds its place and is combined holistically with other senses. The second foundation is a concrete cultural and historical lifeworld, upon which the space of sense constructed. Similar ideas can also be found at work in Sellars and Searle.