 In his earlier philosophy of science, Whitehead revolted against the bifurcation of nature into the worlds of primary and secondary qualities, and he promoted the harmonization of the abstractions of mathematical physics with those of Hume’s sensationalist empiricism, as well as the inclusion of more concrete intuitions offered by our perception—our intuitions of causality, extension, cogredience, congruence, color, sound, smell, etc. Closely linked to this completion of the scientific scheme of thought, Whitehead developed a new scientific ontology and a new theory of perception. His scientific ontology is one of internally related events (instead of merely externally related bits of matter). His theory of perception (cf. Symbolism: its Meaning and Effect) holds that our perception is always perception in the mixed mode of symbolic reference, which usually involves a symbolic reference of what is given in the pure mode of presentational immediacy to what is given in the pure mode of causal efficacy: