Most things are complicated – even things that seem simple. You wouldn’t be shocked to learn that modern cars or computers or air traffic control systems are complicated. But what about, say, toilets?

If you take a minute and try to explain what happens when you flush one, do you even know the general principle that governs its operation? It turns out that most people don’t. Nobody could be a master of every facet of even a single thing. Even the simplest objects require complex webs of knowledge to manufacture and use. Most people can’t tell you how a coffee maker works, or how glue holds paper together, let alone something as complex as love.

Our point is not that people are ignorant. It’s that people are more ignorant than they think they are. We all suffer, to a greater or lesser extent, from an illusion of understanding, an illusion that we understand how things work when, in fact, our understanding is meagre.

We all have domains in which we are experts, in which we know a lot in exquisite detail. But on most subjects we connect only abstract bits of information, and what we know is little more than a feeling of understanding we can’t really unpack.

So how can we get around, sound knowledgeable and take ourselves seriously while understanding only a tiny fraction of what there is to know?

The answer is that we do so by living a lie. We tell ourselves that we understand what’s going on, that our opinions are justified by our knowledge and that our actions are grounded in justified beliefs, even though they are not. We tolerate complexity by failing to recognise it. That’s the illusion of understanding.

So how can humanity achieve so much when people are so ignorant? It turns out we have been very successful at dividing up our cognitive labour. We would not be such competent thinkers if we had to rely only on the limited knowledge stored in our heads and our facility for causal reasoning. The secret to our success is that we live in a world in which knowledge is all around us.

We have access to huge amounts of knowledge that sit in other people’s heads: we have experts that we can contact to, say, fix our dishwasher when it breaks down for the umpteenth time. We have professors and talking heads to inform us about events and how things work. We have books, and we have the richest source of information of all time at our fingertips, the internet.

But sharing skills and knowledge is more sophisticated than it sounds. Human beings don’t merely make individual contributions to a project, like machines operating in an assembly line. Rather, we are able to work together, aware of others and what they are trying to accomplish. We pay attention together and we share goals. In the language of cognitive science, we share intentionality. This is a form of collaboration that you don’t see in other animals. We actually enjoy sharing our mind space with others. In one form, it’s called playing.

The nature of thought is to draw on knowledge wherever it can be found, inside and outside our own heads. But we live under the knowledge illusion because we fail to draw an accurate line between what is inside and outside our heads. And we fail because there is no sharp line. So we don’t know what we don’t know. What we need is a greater appreciation of how much of our own knowledge depends on the things and people around us. What goes on between our ears is extraordinary, but it ultimately depends on what goes on elsewhere.

The Knowledge Illusion by Steven Sloman and Philip Fernbach (Macmillan, £18.99) is out now. Buy it for £16.14 at bookshop.theguardian.com