A possibility to put a question of truth of knowledge in biological systematics is studied. It is shown that the problem of truth in reference to systematics is wider than a question of classified information reliability. Prerequisites needed for logically accurate formulation of a definition and criteria of truth are considered. It is shown that such prerequisites are present in taxonomic practice, namely in a process of diagnosis compiling. Philosophical analysis of this work has been carried out. Interpretation of an essence of systematics as classification is connected with use of classical concept of truth (which defines truth as correspondence between knowledge and object) in its undeveloped form. Carried analysis allows supposing that a theory of systematics based on diagnostics rather than on classification would be more prospective. Use of imperfect concept of truth can be seen also in notions that system of taxa must reflect its evolutionary history. Development and modernization of Aristotle's orientation to discovery of the object form can become an alternative to such opinions. An aspiration to achieve the truth is the main motive of systematic work. An influence of this aspiration on a selection of purposes of taxonomic work and theoretical comprehension of its bases is shown. Such features of modern biological systematics as its accessibility for new results, criticism in respect of external morphological characters, and interest in intraspecific variability are connected with this aspiration. This motive comes into contradiction with a tendency to withdraw the problem of truth as such, which takes place in some brunches of theoretic systematics.