In an episode of the 1980s British TV series 'Tales of the Unexpected', an amateur botanist discovers that plants emit agonised screams when pruned, though at frequencies well beyond human hearing. So overcome is he with sympathy for this suffering vegetation, and so apparently bizarre his demands that a local doctor give medical attention to his trees, that he is quickly packed off to an asylum.

A preposterous flight of fancy from Roald Dahl's ever-fertile imagination? No doubt. But this conceit also raises a profoundly serious point: we have next to no idea which things in the world around us are conscious and which are not.

This might seem like an abstract philosophical issue, but on the contrary, consciousness is the substrate of all suffering and pleasure, and thus the mediator of everything truly important to us. If there were no subjective experience then there would be no such things as kindness, love or joy. However sublime our universe, it would be inconsequential without a consciousness to perceive it. True, in a world with no subjective experience there would also be no cruelty, pain or worrying (including that of the kind I am doing now). But this is precisely the point: how are we to maximise happiness and minimise suffering if we do not reliably know where and when they can exist?

Our usual rule of thumb for the presence of consciousness is to assess, based on superficial cues, how similar to ourselves something appears to be. Thus a dog is more conscious than a duck, which in turn is more conscious than a daffodil. But our intuitions about so many things—rom the motions of celestial objects to the likelihood of winning the lottery—are so often wrong that we are foolish to rely on them for something as important as the ultimate source of all joy and strife.

Famously, and ironically, the only thing of which we can be truly certain is the existence of our own subjective experience, and we see the physical world only through this dark glass. Yet the scientific method has proved a remarkable tool for clarifying our view and enabling us to develop an elaborate, apparently objective consensus about how the world works. Unfortunately, having provided us with an escape route from our own subjectivity, science leaves us almost completely impotent to probe the nature and origins of subjective experience itself. The truth is that we have no idea what things have consciousness, where it comes from, or even what it is. All we really know is how it feels.

That may sound like an odd statement to come from a former neuroscientist. It is certainly true that we understand an impressive amount about how the brain functions as a physical system, and also about the ways in which different brain states correspond to various reported subjective experiences. But this is a very long way from understanding consciousness well enough to be able to do what really matters: determine with reasonable certainty what does and does not possess it, and to what degree.

This is a fabulously hard task. Daniel Dennett's outstanding book, 'Consciousness Explained', for all its considerable eloquence and erudition, falls well short of the claim of its title. Indeed, we have tied ourselves in such intellectual knots over consciousness that it is hard to discern more than incremental progress since Descartes.

For an example of the difficulties involved, consider John Searle's celebrated Chinese Room thought experiment, which purports to show— contrary to Alan Turing's claims—that input-output characteristics alone are insufficient to determine the existence of a conscious mind. Intuitively this conclusion seems right: a sleeping person, immobile and inattentive, might nevertheless be experiencing vivid dreams. Conversely, I can drive a familiar route without forming any conscious record of the journey that took me to my destination. But the Chinese Room does nothing to prove this thesis, for it is a thought experiment and the trouble with thought experiments is that the researcher chooses not only the experimental conditions but also the results. This makes them useful for testing the internal consistency of ideas but almost useless for probing mysterious, apparently emergent phenomena like consciousness. (To see this, carry out the same thought experiment on the 3kg lump of electrophysiological goo called the human brain and if you're being consistent then you'll get the same result: there appears to be no conscious understanding anywhere inside.)

Over the last decade or two, neuroscientists have at last shed their qualms about investigating the mysterious and transcendent phenomenon of consciousness, with some interesting results. Our best guess these days is that consciousness arises when certain types and quantities of information are integrated in the brain in certain ways. But this is still very hand-wavy, not only because the parameters are so ill-defined, but also because we don't even know exactly what we mean by information. After all, every physical system contains 'information' of one sort or another, and 'computes' its own behaviour, so the brain is far from unique in this respect.

What's more, even if we understood exactly what kind of information has to be brought together in precisely what combinations and quantities to ignite a spark of consciousness, we'd still be in the dark as to whether this is an emergent or fundamental phenomenon, and why this physical universe even allows subjective experience to exist at all. Those mysteries remain as dark to us as the nature of existence itself.

Towards the end of 2012 doctors picked up the first message (via a brain scan) from a patient in a persistent vegetative state. But if input-output characteristics can't be trusted—and they probably can't—then we really have no way of confirming whether this represented a conscious act or an insensible physical response. Similarly, what of patients under anaesthetic? For all we know they may be suffering in agony during their operations even if they have no memory of it afterwards. And so on and on, from human embryos to birds, insects and, yes, even plants. Not to mention all those computers we're bringing into existence: might they have an inner life too?

It is possible that we are rare, fleeting specks of awareness in an unfeeling cosmic desert, the only witnesses to its wonder. It is also possible that we are living in a universal sea of sentience, surrounded by ecstasy and strife that is open to our influence. Sensible beings that we are, both possibilities should worry us.