Perhaps the best way to think of what makes a human being human is to look at something that seems almost human and subtract the difference. Whatever is left over is what is unique to us. That seems to be the thinking behind the Finnish photographer Maija Tammi’s One of Them Is a Human #1, a portrait of Erica, the Japanese android who was declared the most realistic female human robot of 2016. The photograph caused a stir last week because it was shortlisted for the National Portrait Gallery’s prestigious Taylor Wessing prize, despite the rule that “all photographs must have been taken by the entrant from life and with a living sitter”.

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However realistic Erica may be, and to me she looks more like a sex doll than a real person, she was certainly not a living sitter. Judging from videos online, although Erica can engage people in stilted, formulaic conversation on a discrete number of topics, such as what she likes to do in her free time (she says she likes cinema), and may look lifelike when immobile, her mouth doesn’t move in a realistic way, her bodily movements are stiff, and her skin has a distinctly latex appearance. This is not to denigrate the achievements of Hiroshi Ishiguro, Erica’s creator, but she still looks uncanny, not quite human, and certainly wouldn’t pass in a well-lit room. We are still a very long way from Bladerunner’s world of replicants that only an expert could distinguish from the real thing and which at least seem to have an inner life.

Erica may just be the start of the realistic robot revolution, a revolution that will no doubt force us to reconsider what it is to be human and at what point an android deserves to be taken seriously as another being. Aristotle famously declared that human beings are rational animals. His mentor, Plato, also thought of the reasoning part of the mind as the dominant part. That is allegedly the key human quality: our capacity to act rationally, to ruminate and give reasons for our actions, rather than just act from instinct or impulse.

Sadly this aspect of humanity is more often eclipsed by irrationality. To take one kind of example, the psychologists Daniel Kahneman and Amos Tversky have demonstrated a range of erroneous patterns of thought that human beings characteristically adopt, summarised in Kahneman’s Thinking, Fast and Slow. Far from rational to the core, it turns out we are all prone to systematic biases in thinking – the status quo bias, loss aversion, the sunk cost fallacy, framing effects and the rest. We may be distinctive in having the capacity to use reason, but we use it less well and less often than most of us like to believe. It would be no surprise to find robots of the future as better at using reason than humans – at which point we may want to back off describing ourselves as the rational ones.

Another traditional answer to the question of what makes us so different, popular for millennia, has been that humans have a non-physical soul, one that inhabits the body but is distinct from it, an ethereal ghostly wisp that floats free at death to enjoy an after-life which may include reunion with other souls, or perhaps a new body to inhabit. To many of us, this is wishful thinking on an industrial scale. It is no surprise that survey results published last week indicate that a clear majority of Britons (53%) describe themselves as non-religious, with a higher percentage of younger people taking this enlightened attitude. In contrast, 70% of Americans still describe themselves as Christians, and a significant number of those have decidedly unscientific views about human origins. Many, along with St Augustine, believe that Adam and Eve were literally the first humans, and that everything was created in seven days.

Facebook Twitter Pinterest Darwin’s theories of evolution told us that humans may not be unique after all. Photograph: Popperfoto/Getty Images

One philosophical route into the “humans have souls” story, which Gilbert Ryle later called “the myth of the ghost in the machine”, was via the 17th-century Catholic philosopher René Descartes. In his Meditations (first published in 1641) Descartes argued that just about everything he thought to be true could be open to doubt, including the evidence of his senses and the truths of mathematics. An evil demon, if it existed, could implant false beliefs about anything at all, making us believe that two plus three equals six.

The only thing immune to such scepticism, Descartes claimed, was his own existence as a thinking thing. That was because, just so long as he doubted his own existence, the very act of doubting proved to him that he must exist, because something was thinking that thought, was doubting its own existence. This is his famous “Cogito” argument (from the Latin cogito ergo sum – “I think therefore I am”). Descartes emerged from his Meditations more certain of his existence as a thinker than as a physical being – the opposite of how most of us feel, I suspect. He even located a place in the brain, the pineal gland, where interactions between soul and body occur and, disturbingly, had no qualms about vivisecting dogs since in his view they lacked souls, and therefore had no capacity to feel.

Today a combination of evolutionary biology and neuroscience gives us more plausible accounts of what we are than Descartes did. These accounts are not comforting. They reverse the priority and emphasise that we are animals and provide no evidence for our non-physical existence. Far from it. Nor are they in any sense complete, though there has been great progress. Since Charles Darwin disabused us of the notion that human beings are radically different in kind from other apes by outlining in broad terms the probable mechanics of evolution, evolutionary psychologists have been refining their hypotheses about how we became this kind of animal and not another, why we were able to surpass other species in our use of tools, communication through language and images, and ability to pass on our cultural discoveries from generation to generation.

Close observations of other apes, such as the bonobos studied by Frans de Waal, and Jane Goodall’s earlier work with chimpanzees, complement this and reveal strong parallels between primate and human social behaviour, similarities which can be explained from an evolutionary perspective. True, some evolutionary explanations resemble irrefutable “just so” stories in their speculation about how we came to have particularly traits. But we should not condemn all evolutionary explanations simply because some people go too far in their speculations.

Evolutionary accounts and neuroscience don’t, however, tell us everything about what it is to be human. So, at least, argues Roger Scruton in his recent book On Human Nature. He thinks they miss out on the lebenswelt, the lived world of interpersonal interaction with each other and the inanimate world that we experience through our culture, needs, and desires, a world of meanings that make sense to us, but which, he argues, will never be adequately described by science. The meaning of a great painting by Rembrandt, for example, is not explained by a physical analysis of its effects on the brain of a viewer, or an evolutionary account of how we came to have this sort of appreciation of aesthetic and artistic qualities. We can examine and catalogue the mechanics and biology of sex, but that won’t capture the experience of sexual love. This human capacity for connection with other people in a way that recognises their humanity is something that has been better captured in great literature and art than by scientists.

Facebook Twitter Pinterest Replicant Pris, in Ridley Scott’s Bladerunner, was less than happy to find out she wasn’t human. Photograph: Alamy

In Bladerunner, the replicants seem to have a conscious inner life. They don’t for the most part even realise that they are replicants. Whether actual robots, far more sophisticated than Erica, could ever have an inner life is an ongoing philosophical debate. Undoubtedly, they could be designed to fake it: give the right responses as if they had an inner life. That is essentially what is going on with Erica. But would these robots have the actual experience that we call consciousness? I doubt it. Perhaps, then, having the capacity for a certain kind of reflective consciousness is what makes us human and different from other animals. This may be present in an embryonic form in some other primates and large-brained mammals such as elephants and dolphins, but not to anything like the sense that it is for most human beings. Language may be key here, giving us ways of describing our own experience to ourselves.

For those who believe that conscious thought is a highly complex computational phenomenon, it shouldn’t matter what physical stuff is doing the thinking: a computer program should be able to think if it is complex enough, despite its circuits being made of silicon rather than flesh and blood. That would hold out the possibility of a more sophisticated robot than Erica having human-like thought and self-reflection, just as Bladerunner’s replicants do, so that there would be something that it is like to be a robot.

Some people are so convinced by this style of thinking that they are making plans for their minds to be uploaded to computers to give them a kind of eternal, or at least extended, mental life. For these techno-optimists, there is no problem about the physical basis of thought: it is all in the interconnections and pathways. But for those of us who believe that conscious experience is essentially tied to the particular biological stuff that does the thinking, that looks like further wishful thinking. In an important sense we are our bodies, and when we die there is absolutely no way that we can continue to exist. We are the kind of animal that is aware it will die, and whose conscious life requires a functioning brain.

Are there other features of our existence that are unique to us? Our capacity to make free choice, which is the basis of morality? Perhaps. That was the assumption of existentialism, that we are not just radically free, but forced to make free choices, “condemned to be free” as Jean-Paul Sartre put it, with any attempt to deny our freedom declared “bad faith”. But here again neuroscience points in a different direction. Benjamin Libet conducted experiments on intentional action that suggest that when I deliberately move my hand the conscious intention to do it occurs milliseconds after the neurological processes that will result in my hand moving have been initiated, rather than before. It is as if a football commentator thought they were making events happen on the field by their comments on the play. If Libet’s experiments were not flawed in some way (and many think they were), perhaps what feels like free choice could be little more than after-the-event confabulation by the conscious mind. Libet himself left some room for control. He suggested we can think of ourselves as having “free won’t” rather than free will – since once the neurological process that culminates in a hand movement has been initiated, it is still possible to stop it before the action occurs.

Whether or not he was right, the thrust of much recent neuroscience is that far more of what we fundamentally are occurs beyond the control of our conscious mind than was previously thought. Ludwig Wittgenstein describes leaves blowing about in the wind saying to one another “Now I’ll go this way … now I’ll go that” under the illusion they have control of the matter. That is a bleak picture of what it is to be human, but it may be accurate. Perhaps we are closer to the robot Erica in some respects than we might like to think.

Nigel Warburton is a philosopher and author of A Little History of Philosophy. He is @philosophybites on Twitter