(From Diagnosis 2757 to discern, or distinguish,) also dignotio. It generally means distinction, and is generally confined to diseases; therefore, diagnostics mean the signs of diseases by which they may be known and distinguished. They are of two kinds, viz. the adjunct, and pathognomonic: the first are common to several diseases, and serve only to point out the difference between diseases of the same species; the latter are those which always attend the disease, and distinguish it from all others.

The knowledge of diagnostics has always been considered as of the greatest importance in the practice of medicine; and to know a disease is justly believed to be the first step in its proper treatment. The want of accuracy in distinction is a source of much uncertainty-; and it is not uncommon for those who practise from the popular treatises, to apply the remedies recommended for one complaint to those of another; so very imperfect are such works in discrimination. Distinction is the greatest object of nosological systems; and this advantage we shall afterwards have occasion to point out. See Nosology.