Flies Do It, Leeches Do It— Even Biologists Do It: Free Will Explained

Do both of these structures contain the capacity to exercise free will?

Philosophers shop for free will as hypochondriacs do for good health. Nothing but the real thing will do, and yet they refuse to trust the countless everyday indications that they already possess their quarry. Of course it seems to be the case that to act on one’s decisions is to exercise one’s freedom, but can it be true that, winding time back to the crucial moment, you or I could have done otherwise?

Enter the biologists. We can account for free will so long as we are willing to share it with flies, leeches, and all forms of life that enjoy a nervous system. As Bjorn Brembs has recently argued in The Royal Society, we should equate free will with variability, or an organism’s power to determine the precise way in which it responds to its environment. Variability, Brembs contends, is a (as yet little understood) neural process that amplifies random fluctuations in the brain in order to introduce non-sensory dependent variations into an organism’s behaviour.

Brembs finds strong evidence for variability in the behaviour of fruit flies and leeches. If a fly is exposed to a moving grate pattern, it will shift its head in order to remain facing the same direction as the grating (thus stabilising what it sees). But the sensory input does not seem to determine the exact movements of the fly’s head; experimentation has shown that there is not enough variability in the neurons that respond to the visual stimulus in order to determine the extra movements of the head. Some process downstream in the fly’s brain introduces variation in its behaviour, without taking its cue from sensory stimuli. Meaning that if the fly had that moment of the experiment to live over (leaving everything in its sensory environment unchanged), it may indeed have behaved differently.

Leeches, meanwhile, will exercise their right to crawl or swim in spite of receiving invariant electrical stimuli intended to produce only crawling behaviour. As the experimenters put it, under “carefully controlled experimental circumstances, the animal behaves as it damned well pleases”.

Brembs sees this capacity to escape stimulus-determined behaviour as delivering onto all animals enough free will to avoid the dangers that come with wholly predictable behaviour, and perhaps also the despair of believing that you have yet to actually make any free choices in your life (nor, by cosmic fiat, can you ever do so). But will you feel free merely by knowing that every time you reach for your mobile phone, your brain will introduce novel variations into the trajectory of your hand?

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