Empiricism is the philosophical doctrine that knowledge and understanding originate in experience, especially sensory experience. Thus, the empiricist philosopher John Locke likened the mind of an infant to a "blank slate" that contained nothing but the potential to register the facts and concepts that the experiences of a lifetime would write on it. Of ancient provenance, empiricism was formalized during the 17th century and is a major force in modern ideology. Its most significant expression is experimental science, with an experiment being a more or less controlled experience. Empiricism is the epistemology of science; it specifies the grounds on which most scientists and geographers justify claims to factual knowledge.
Empiricism takes weak and strong forms. The weak form is the everyday habit of using sense data to answer questions about contingent facts. If one wishes to know whether or not there is milk in the refrigerator, for instance, one probably will look in the refrigerator. The question could be answered deductively (today is Wednesday, one's spouse shops on Tuesday, etc.), but such reasoning could lead at best to supposition. A weak empiricist believes that looking is the only way in which to verify or have positive knowledge that milk, or anything else, is present; hence, empiricism is also called verificationism (or positivism). Weak empiricism is very common in human geography. Strong empiricism elevates this method to the epistemological doctrine that experience not only is a ground for knowledge but also is the best or only ground. There are not, and cannot be, meaningful assertions other than those verified, or verifiable, by positive empirical evidence. This is a revolutionary doctrine because its demand that all alleged truths be put to the test of experience undermines metaphysical beliefs grounded in intuition, pure reason, faith, tradition, and authority. Doctrinal empiricism sometimes is content to invalidate the epistemological claims of these alternative routes to knowledge and pronounce itself agnostic with respect to the objects these methods allegedly apprehend (e.g., God, value, beauty, freedom, the soul). Frequently, however, strong empiricism affirms the ontological doctrine that extraempirical, nonobservable things do not exist. Advocates call this ontological doctrine naturalism, whereas dissenters call it scientism or positivism.
Dissenters come in many varieties but share a belief that there is a knowable reality that exceeds and conditions the phenomena of sensory experience. This metaphysical or transcendental realm is not one that experimental science will one day discover with further investigations and improved techniques because it can be apprehended only by nonempirical methods. Antiempiricists sometimes are called rationalists because they believe that the mind (or at least the acute minds of rational souls) can directly apprehend transcendental truths intuitively and without the aid of the senses. (The name is confusing because their intuitions and inferences often affirm the very beings— God, the soul, value—that empiricists expunged in the name of rationalization.) In contemporary human geography, rationalist antiempiricism usually is called antipositivism, and it is evident in humanistic introspection and the theorizing of social theorists.
Curiously, one might say that the limitations of empiricism (and the naturalistic ontology that follows from it) have grown apparent as empiricism has prospered— as more and more nonempirical truths have been put to the test of experience, failed, and been discarded. The radical doctrine of empiricism does not verify the objective existence of values such as justice, beauty, and the good, and so it removes these rational ends from human calculation even as it, through empirical investigations, engrosses the means that humans may employ to achieve what now appear as arbitrary and emotive ends. Thus, widespread adoption of the doctrine of empiricism at least partly explains a postmodern predicament, that is, our endlessly increasing ability to do just about everything except agree just what it is we ought to do.