He had no acquaintances at the party, and was not really an extrovert. The food was not tasty, the music insipid, and thus – forced to stay by external circumstances, and not wanting to give the impression of being idle – he turned to the single occupation he found worth trying: looking at people's behavior. How they chose to dress, what they chose to eat, where they chose to sit. It was entertaining – just entertaining, he was aware he would probably miss in all his conclusions – to try to discover their motivations, the root of their choices. Well… as if such thing as having a choice ever existed. He smiled in amusement thinking about Buridan's ass.

Buridan's ass is a thought experiment, dating back to medieval times, devised precisely to discuss whether a choice is always based on a rational decision. It presents an ass, equally thirsty and hungry, and equally distanced from a pile of hay and a bucket of water. Since the animal cannot find a reason to prefer one option to the other, unable to take action, he dies, of both hunger and thirst. The thought experiment, that exposes the complex nature of free will by reducing its assumption to an absurd situation, is framed in a (perhaps endless) discussion that for centuries was the battleground of philosophers, now joined by neuroscientists.

For an experimental neuroscientist, analyzing behavior is a phenomenal task. The patterns behind choices and actions are woven up from tens of millions of interconnected neurons, whose firing activity takes place in a millisecond timescale. To understand in full any of those events, to lay any causal connection between them, it is necessary to have certain control of the neuronal activity, exciting and inhibiting it on demand. An example of an external tool for such control is the use of electrodes, but those may cause unintentional firing in the surroundings of the targeted neuron. Luckily for researchers, there exist algae and bacteria that find in sunlight the reason to excite or inhibit their behavior.