The present paper is an exposition of the organic thought in Whitehead’s Science and the Modern World with detailed discussions of the major philosophical issues involved. Whitehead’s Science and the Modern World marks the beginning of the transition in his philosophical development, the transition from natural philosophy to metaphysics. Before the publication of the book, Whitehead made the philosophy of nature the main theme of his studies by a deliberate demarcation between natural philosophy and metaphysics, which suggests that metaphysics is beyond the scope of the philosophy of nature. The publication of Science and the Modern World indicates his changing attitude toward metaphysics and his inclusion of natural philosophy into the sphere of metaphysics, which is fundamental to his later development of speculative cosmology. In the Preface to the book, Whitehead reconsidered the function of philosophy as the critic of cosmologies, that is “to harmonize, re-fashion and justify divergent intuitions as to the nature of things,” and as the critic of abstractions, such as highly abstract scientific ideas, so that justice can be done to our concrete experience. It seemed to Whitehead that in our concrete experience there are metaphysical and religious experiences besides scientific understanding and that philosophical inquiry should no longer be confined to “the concept of nature,” but extend to axiology and metaphysics. Largely following the major themes of his philosophy of nature—the criticism of Scientific Materialism and its related doctrines and his proposals for the doctrines of “immediate experience,” pluralist realism, organicsm, and process—Whitehead considers all this in the outline of the present book. Inundation, he substitutes the concept of “organism” for the concept of “matter” as “the ultimate fact of nature,” and the “prehensive character” for the “simple-located character” of space and time. Finally, he constructs a theory of organic mechanism as a replacement for material mechanism. And by the sway of his interest in metaphysics, Whitehead acknowledges that all fact, as actuality with intrinsic reality, has value. This very interest also goaded him to develop his organic theory of nature into a descriptive metaphysics that has God as its ultimate presupposition. In a word, there are at least three philosophical endeavors that characterized Science and the Modern World: the construction of an organic mechanism based on prehensive events, the emphasis on the idea of value and of God. All this can be regarded as a preparation for the publication of Process and Reality, a more elaborated system of metaphysics and speculative cosmology.