Empiricism is a philosophical theory which argues that human knowledge is derived entirely from sensory experience. As a branch of epistemology, empiricism disregards the concept of instinctive ideas and focuses entirely on experience and evidence as it relates to sensory perception. Ferociously debated, the philosophy of empiricism eventually spawned additional schools which would take it to different levels of application and direction.
In the 1600s, as a response to the rationalism theory fiercely defended by Rene Descartes, the philosophy of empiricism was first put forth in John Locke's An Essay Concerning Human Understanding. Locke argued that the only way by which humans acquire knowledge is through experience. Expanding on Aristotle's notion of humans as blank slates, Locke firmly argued that humans are incapable of formulating or possessing inherent ideas.
Continental rationalism, empiricism's rival theory, stated that everything possesses some kind of explanation. According to the rationalists, sensory experiences did not belong in the acquisition of knowledge. To followers of this theory, knowledge was only obtained through substantive thought and only through this intellectual perception could humans gain understanding.
Irish philosopher George Berkeley was an idealist who believed that Locke's philosophical theory was dangerously atheistic. He responded to Locke's theory in the early 1700s with his Treatise Concerning the Principles of Human Knowledge, where he asserted that material things are composed exclusively of ideas, which is naturally a mental process. Berkeley imposed a religious tone to his version of empirical theory, referred to as subjective idealism, which stated that all that humans perceive is the idiom of God.
David Hume, a Scottish philosopher, was a staunch supporter of Locke and argued that human understanding is solely derived from sensory experience. Hume stated that knowledge is either a matter of fact, such as through direct observation, or related to an idea or theory, as applied in logic and mathematics. According to Hume, the external, physical world is not something which can be rationally deciphered or justified.
Locke, Berkeley, and Hume's notions of empirical theory remained pure until the early 1900s, when additional schools of empirical thought were established. Phenomenalism was an intense approach to empiricist theory which stated that all physical objects could be reduced to mental objects and that, ultimately, only mental objects exist. As expressed by John Stuart Mill in the late 1800s, phenomenalism was closely related to Berkeley's subjective idealism in theory and application.
Logical empiricism, also referred to as logical positivism, became a movement which attempted to combine empirical and rational thought. Science and metaphysical thought influenced logical empiricism by way of the marriage of the importance of nature and the existence of matter. The school of logical empiricism was committed to the unification of the sciences in that it argued that all scientific hypotheses should be expressed in common language for the sake of better understanding and presentation.