The Early and the Later Wittgenstein, Contrasted
Wittgenstein's early philosophy crystalized in his "Tractatus Logico-Philosophicus," more commonly just "the Tractatus." The book was largely conceived and executed by Wittgenstein when he was a POW held by the Italians in World War I. At Cambridge University, the two most eminent philosophers in the English-speaking world at the time, Bertrand Russell and G.E. Moore, were so impressed by the work that they awarded the Ph.D to Wittgenstein as if it were written as a doctoral dissertation, waiving virtually all of the other conventional requirements for the degree. Wittgenstein told them that they did not understand his work, and they agreed.
In the Tractatus, Wittgenstein laid out a "picture theory" of logic and language. In relation to the actual world, words stand as pictures of the concepts that they represent. This by itself seems fairly trivial, but let me use a picture of my own to try to explain what made his approach seem both radical and entirely new.
Picture a globe, a globe of the earth that is, approximately 20 feet in diameter. It is made of the thinnest titanium, such that the inside of the globe shows with perfect definition the contours of the outside of the globe. Those contours include all of the elevations on the surface, from the Marianas Trench to the summit of Mt. Everest.
Inside the globe, at its center, stands a little man, Harry Homunculus, with a long stick. The length of the stick is such that Harry can trace, can define, every bit of the surface of the world from within as it were. He proceeds to do this.
But what Harry is actually delineating are the limits of our logic and the reach of our language as a vehicle for drawing all of those pictures of what is happening in the world.
This picture of Harry is a picture of what Wittgenstein purports to be doing in his book -- laying out the limits of logic and language as they engage with, and describe, the world. Critically, however, the most important aspects of human life -- everything spiritual, the joy we find in works of art and in our most important relationships, the mystery of our mortality -- all lie outside the boundaries of the globe. That is not to say that we do not attempt to talk about them. In fact, the problems in philosophy that we have wrestled with for thousands of years can be seen as arising out of our futile attempts to try to address these things as if they were as susceptible to description and explanation as, for example, the disposition of two automobiles that we find on the road after a collision. When we try to talk about them in this way, what comes out is "nonsense." In a sense we can point at them, but we should not utter "propositions" about them, because the realm of propositions should be left for things that exist in the world. "Whereof we cannot speak, thereof we must remain silent."
It is a remarkable fact that the Tractatus was viewed as the most provocative and original philosophical achievement of the early 20th century, that its views were not systematically challenged in any way, but that Wittgenstein himself concluded, by the early 1930s, that they were fundamentally wrong! His later philosophy, which dismantled the first, albeit in an oblique way, found its way into print only after his death in 1951, and largely in the form of books on particular themes, like "Remarks on Colour," that were pieced together literally from scraps of paper by his "disciples" and literary executors. (He despised the fact that he had disciples, in the sense of lesser minds who spread his gospel, but without thinking rigorously for themselves in the process.) The exception to composition from scraps by others was "Philosophical Investigations," which he himself compiled but which was not published in his lifetime. "The Investigations" or "PI" should probably be viewed as the most important work of philosophy of the entire 20th century.
In what way did Wittgenstein depart from his earlier views in PI and related texts?
A fundamental new insight, derived from examining how language is actually used in the world by real people as opposed to philosophers, was that concepts are fluid; their edges are soft and shifting. This does not mean that we must be sloppy in our thinking. The concepts function just fine notwithstanding the fact that they are not "atomic." We distinguish between the river and the river bed, and the difference is important, but we cannot describe a fixed boundary between the two. So Harry the Homunculus cannot do what he set out to do. It was impossible in principle for him to lay out a fixed boundary for a world within which we utter propositions consisting of concepts that themselves have fixed boundaries.
One radical consequence of this view is that it obviates the need to search for an "inner" mechanism, in the brain presumably, that accounts for, and mirrors, our outer facility with language and logic. It is anti-Chomskian, and indeed rather scary in its insinuation that there is nothing beneath the surface, that, as Wittgenstein said in so many words, "nothing is hidden." To the extent that there is an explanation for how we are able to use language, it is an "anthropological" one. Language resides in "forms of life;" it is embedded in our culture. No explanation beyond this is required. Indeed, philosophers' attempts to come up with such explanations simply have led us into confusion and error.
The view has important consequences as well for artificial intelligence. Wittgenstein would never say, I think, that even the most sophisticated computer could evolve into consciousness, or that Elon Musk could cheat death by uploading his mind into a machine. (It is perhaps noteworthy that Wittgenstein's time at Cambridge overlapped to some extent with that of Alan Turing; I believe that they knew each other, how well it's hard to say.) The fact that the most powerful AI natural-language translation engines now extant have no real "semantic component" as it was conceived by Chomsky suggests, if it does not prove, that Wittgenstein was right about the soft and the fluid edges. And likewise our ability to use words and expressions in novel ways and yet be understood. The first person to say, before Stevie Wonder, "you are the apple of my eye," was understood, mirabile dictu.
One early view that Wittgenstein did not abandon was that the most important and profound things in life reside outside philosophy. He admired Kierkegaard, Dostoevsky and Tolstoy in particular, as well as the gospels. In this sense he very much embraced the "inner" life notwithstanding its ineffability; he just did not want to philosophize about it.