“Ice” is a meaningless term on the scale of individual molecules. Looking only at the parts would be like trying to figure out how water freezes by studying a single water molecule. They argue that we will never truly understand how the mind emerges from our nervous system if we break the brain down into disconnected pieces. Yet some neuroscientists think it is time to tackle the challenge. Add 100 billion more neurons to the problem, and the endeavor turns into a cosmic headache. The interactions between just a few neurons can be a confusing thicket of feedbacks.

Their reticence stems, in part, from the sheer scope of the challenge. But few have tried to contemplate the brain on its many scales at once. Others might look at the brain at an even higher scale, observing all the regions that become active when we perform a particular task, such as reading or feeling fear. Others choose a higher scale: they might, for example, look at how the hippocampus, a cluster of millions of neurons, encodes memories. Some neuroscientists dedicate their careers to the workings of individual neurons. It’s the kind of question that neuroscience, for all its triumphs, has been ill equipped to answer. How our minds emerge from our flock of neurons remains deeply mysterious. Join together 100 billion neurons-with 100 trillion connections-and you have yourself a human brain, capable of much, much more. But join together 302 neurons, and they become a nervous system that can keep the worm Caenorhabditis elegans alive-sensing the animal’s surroundings, making decisions and issuing commands to the worm’s body. On its own, in its dish, the neuron can’t do much. Bathe the neuron in various neurotransmitters, and you can alter the strength and timing of its electrical waves. If you deliver pulses of electricity to one end of the cell, the neuron may respond with extra spikes of voltage. From time to time, it spontaneously unleashes a wave of electric current that travels down its length.

A single neuron sits in a petri dish, crackling in lonely contentment.

How many neurons in the human brain