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A physical disorder (as a medical term) is often used as a term in contrast to a mental disorder, in an attempt to differentiate medical disorders which have an available objective mechanical test (such as chemical tests or brain scans), from those disorders which have no objective laboratory or imaging test, and are diagnosed only by behavioral syndrome (such as those in the Diagnostic and Statistical Manual of Mental Disorders or DSM manual. Most familiarly, this is a term used as opposed to supposed "purely" mental disorders.

Differentiating physical from mental disorders is a difficult problem in both medicine and law, most notably because it delves into deep issues, and very old and unresolved arguments in philosophy and religion. Many materialists believe that all mental disorders are physical disorders of some kind, even if tests for them have not yet been developed (and it has been the case that some disorders once widely thought to be purely mental, are known to have physical origins, such as schizophrenia). Some^[who?]psychiatrists take the position that some or all mental disorders may be seen analogously to the information level of programming in a computer. In this case, all such disorders are associated with physical changes in the brain, but the pathology is at the level of brain information and programming (software), which is fundamentally separate from the means to store it.