“Thinking in pictures,” Sigmund Freud once wrote, “stands nearer to unconscious processes than does thinking in words, and is unquestionably older than the latter both ontogenetically and phylogenetically.” There is, in other words, something primordial, something foundational, about thinking visually.

Such a view is anathema to many philoso- phers, a good many of whom believe that all thought is propositional, that to think is to use words. For some of the most distinguished philosophers in history, thinking and verbalis- ing were practically the same thing. Bertrand Russell sometimes to his great frustration, was hopeless at visualising and was more or less indifferent to the visual arts. His mental life seemed almost entirely made up of words rather than images. When his friend Rupert Crawshay-Williams once gave him an intelligence test that involved matching increasingly complicated geometrical shapes, Russell did extremely well up to a certain point and then exceptionally badly after that. “What happened?” Crawshay-Williams asked. “I hadn’t got any names for the shapes,” Russell replied.

In this, as in many other respects, Ludwig Wittgenstein was Russell’s opposite. For Wittgenstein, to think, to understand, was first and foremost to picture. In conversation with his friends, he several times referred to himself as a “disciple” or “follower” of Freud and many people since have been extremely puzzled what he might have meant by this. I think Freud’s remark quoted above might provide the key here, that it might have something to do with the emphasis one finds in Freud on the primordiality of “thinking in pictures”.

Like Freud, Wittgenstein took very seriously indeed the idea that our dreams present us with a series of images, the interpretation of which would reveal the thoughts we have relegated to the unconscious parts of our minds. “If Freud’s theory on the interpretation of dreams has anything in it,” Wittgenstein once wrote, “it shows how complicated is the way the human mind represents the facts in pictures. So complicated, so irregular is the way they are represented that we can barely call it representation any longer.”

It was fundamental to Wittgenstein’s think- ing – both in his early work Tractatus Logico-Philosophicus and in his later work Philosophical Investigations – that not everything we can see and therefore not everything we can mentally grasp can be put into words. In the Tractatus, this appears as the distinction between what can be said and what has to be shown. “Whereof one cannot speak, thereof one must be silent,” runs the famed last sentence of the book but, as Wittgenstein made clear in private conversation and correspondence, he believed those things about which we have to be silent to be the most important. (Compare this with the logical positivist Otto Neurath, who, echoing Wittgenstein, declared: “We must indeed be silent – but not about anything.”)

To grasp these important things, we need not to reason verbally, but rather to look more attentively at what lies before us. “Don’t think, look!” Wittgenstein urges in Philosophical Investigations. Philosophical confusion, he maintained, had its roots not in the relatively superficial thinking expressed by words but in that deeper territory studied by Freud, the pictorial thinking that lies in our unconscious and is expressed only involuntarily in, for example, our dreams, our doodles and in our “Freudian slips”. “A picture held us captive,” Wittgenstein says in the Investigations, and it is, he thinks, his job as a philosopher not to argue for or against the truth of this or that proposition but rather to delve deeper and substitute one picture for another. In other words, he conceived it as his task to make us, or at least to enable us, to see things differently.

The importance Wittgenstein attached to seeing was vividly portrayed – in an appropriately visual form – in the “Wittgenstein: Philosophy and Photography” exhibition at the London School of Economics earlier this summer and, before that, at the University of Cambridge. The exhibition brought together a range of fascinating photographs that included studio portraits of the Wittgenstein family (he had four brothers and three sisters) in their palatial homes in Vienna; pictures of Wittgenstein himself as, in turn, a baby, a navy-suited young boy, a student, a soldier and finally a professor; photographs of the modernist house he designed in Vienna for his sister Gretl; holiday snapshots that Wittgenstein took on a cheap camera he had bought in Woolworths; pages from his photo album containing tiny pictures of his friends and family members; and a series of (frankly rather weird) photographs that Wittgenstein took in a photo booth in which he changed his expression and the direction of his eyes after each shot so that the series might be put together in a flip-book that forms the nearest thing we have to moving images of the great philosopher.

The exhibition began with its most intriguing item: a composite photograph made up of four portraits of Wittgenstein and his three sisters (see above). At first, it looks like a picture of a single person, albeit one of indeterminate sex; a very effeminate man perhaps, or else a rather “butch” woman. But then one notices details of the various component photographs. Around the neck, for example, one sees a strange assortment of accessories: Helene’s scarf com- bining oddly with Gretl’s necklace and the ghost of Ludwig’s open-necked shirt. And yet the eyes, the nose and the mouth look like they belong to the same person, enabling one to see directly the very strong family resemblances that existed between these four siblings.

The notion of “family resemblances” is crucial to Wittgenstein’s later philosophy. It plays a critical role in his attempt to unseat the pic- ture that he regards as the root of most philosophical confusion, namely the “Augustinian picture of meaning”. Philosophical Investigations begins with a passage not from a work of philosophy but from an autobiography: St Augustine’s Confessions. In it, Augustine describes how he learned to speak. “When [my elders] named some object,” he says, “I grasped that the thing was called by the sound they uttered”; thus, hearing words used in this way repeatedly, he “gradually learned to understand what objects they signified”.

This passage, Wittgenstein says, gives us “a particular picture of the essence of human language”, a picture that represents meaning as a relationship between a word and an object. This picture is relatively harmless when we confine ourselves to such words as “table”, “chair” and so on but when applied to the more complex notions that philosophers consider – the mind, the soul, justice, truth, meaning – it leads to confusion. We ask, “What is the mind?” and expect the answer to take the form of identifying some thing that the word “mind” refers to.

To overcome this, Wittgenstein suggests we understand words as picking out not some sin- gle thing but a group of things that need not have anything in common. Rather, like members of the same family, they might have a series of similarities and dissimilarities that overlap and criss-cross in various complicated ways. Some Wittgensteins (such as Ludwig and his sisters) might have the same nose, the same mouth, the same eyes but, say, different foreheads. There need not be one thing that all members of the family have in common. Likewise, there need not be any one thing that all instances of the word “truth” have in common. The philoso- phical task of looking for the essence of truth, then, is unending, not because it is deep but because it is an example of the ways in which we can be captured by a picture.

Thus, at the heart of Wittgenstein’s philosophy is what he calls “the understanding which consists in ‘seeing connections’ ”. Here “seeing” is meant not metaphorically, but literally. That is why, towards the end of the book, he devotes so much space to a discussion of the phenomenon of seeing ambiguous figures such as the duck-rabbit. When we “change the aspect” under which we look at the picture, seeing it now as a duck, now as a rabbit, what changes? Not the picture, for that stays the same. What changes is not any object but rather the way we look at it; we see it differently, just as we see a face differently when we look at it, first as an expression of happiness and then as an expression of pride.

“You don’t take enough notice of people’s faces,” Wittgenstein once admonished his friend Maurice Drury. “It is a fault you ought to correct.” The great merit of “Wittgenstein: Philosophy and Photography” was that it provided us with an opportunity to take his advice.

Ray Monk is professor of philosophy at the University of Southampton and the author of "Ludwig Wittgenstein: the Duty of Genius" (Vintage, £12.99)