In the course of his lectures given in Como in 1927 Niels Bohr introduces a very important concept of complementarity. Two years later he publishes his first view of biological questions, which he includes in one series of lectures devoted to quantum mechanics and complementarity.In one of his later articles in the same series, explaining why he started publishing his opinions on biology, he states that this may help understand his interpretation of physics, and especially the category of complementarity. He can see a possibility of using the category of complementarity also outside physics. He hopes that epistemological implications of his category of complementarity can broaden the spectrum of recipients when they will refer to well-known problems, especially in the sphere of biology.Bohr directs his attention to biology hoping to shed new light on the old problem concerning the understanding of the concept of life. He has no doubts that the ‘epistemological lesson’ of quantum mechanics, by drawing attention to the observation conditions in biology, may be helpful in overcoming the controversy between “vitalism” and “mechanicism”. He rejects the vitalistic doctrine with its “vital force” or “entelechy”, as devoid of scientific valor. He also formulates arguments against “mechanicism”.Bohr developed his biological interests directed to philosophy to the end of his life. His last work was devoted to exactly biological questions. Unfortunately, sudden death stopped him from completing it.Because of the fact that in some places Bohr presented his views on biology in an obscure way, some biologists included him in the circle of supporters of vitalism. A more profound analysis of his views proves that this opinion is groundless.

The aim of this paper is to defend the Principle of Sufficient Reason (PSR). I analyse various versions of this principle (and their structure) and various ways of justifying it. Then I present and attempt to challenge some counterexamples allegedly refuting a universal application of the PSR. One can distinguish three versions of the PSR: for each state of affairs there is a sufficient reason for its obtaining (PSR-O); for each true proposition there is a direct or indirect justification (PSR-E); for each state of affairs there exists an intellectual duty to search for a sufficient reason for obtaining of that state of affairs, and for each known true proposition – to search for its justification (PSR-M). There are standard and non-standard versions of the PSR-O. The PSR-Ostand can be only valid if there are no chains of contingent reasons and outcomes with first modules, i.e. all chains are actually infinite. However, there are serious (yet debatable) arguments against the possibility of such infinity. The necessary condition of the PSR-Onon-stand universal working is the existence of a necessary substance (thing): that substance would be a (free) reason of certain contingent states of affairs obtaining in its domain, and those states of affairs would then be reasons for all other contingent state of affairs and things. (The principle does not require a necessary substance to have its own reason). Limiting the chain of reasons to contingent states of affairs ended with a necessary state (if this state is a reason by necessity) would lead to the paradox of universal necessarism. An ontological “cost” of holding the PSR-Onon-stand is a postulate of the existence of the necessary, supra-natural substance, i.e. God. However, there are serious advantages of the view which accepts that the PSR-Onon-stand is generally valid: a nomological unity of the world and explanatory simplicity.