In his Essay 'Is free will an illusion?' (Nature 459, 164–165; 2009), Martin Heisenberg argues that humans must have free will because freedom of action has been demonstrated in other animals — including those as small as fruitflies and bacteria.

Heisenberg's case rests on a combination of random chance and lawfulness, escaping the classic two-horned dilemma of determinism versus indeterminism that is so popular in introductory philosophy courses and textbooks. Starting with William James in 1884, such a two-stage combination of 'free' and then 'will' has frequently been proposed by philosophers and scientists, notably by some quantum physicists after Martin's father, Werner Heisenberg, established irreducible physical randomness with his indeterminacy principle in 1927. But academic philosophers, particularly those who work in the Anglo-American school of analytical language philosophy, have been reluctant to embrace these ideas.

The philosophers' standard argument against free will is simple and logical. If our actions are determined, we are not free. If nature is not determined, then indeterminism is true. Indeterminism implies that our actions are random. If our actions are random, we did not will them.

Heisenberg's proposal makes freedom a normal biological property of most living things, and not a metaphysical mystery or a gift from God to humanity. The genius of this proposal is that it combines randomness with an adequate macroscopic determinism consistent with microscopic quantum mechanics.

John Locke wrote in his Essay Concerning Human Understanding, Book II, that it is not the will that is free but the man. The will determines our actions. Heisenberg writes that Kant would have been pleased. Locke too might have been pleased to see this return to common sense. We may not have metaphysical free will but we do have biophysical free will.