My dog-eared copy of Philosophical Investigations still bears the scars of my 19-year-old student self. Among the underlinings and margin notes are the scribbled phone numbers of former girlfriends now, alas, forgotten: Lynda 2814749, Lucy 2854633, Soni 2845590. From the third year of university, this book was a constant companion. Part notepad, part diary, part address book, but wholly and life-changingly inspirational. Wittgenstein's familiar, intense face peers through a coffee stain from the light green cover, bearing the look of a man ill at ease with the world, like some secular saint whose distance from the rest of humanity is both his gift and his curse.
And yet, the photograph is so out of kilter with what I learned from the book itself. For the photograph invites a sense that there is something absolutely extraordinary going on inside Wittgenstein's head, something unique, something tantalisingly beyond comprehension. After all, that's what philosophers do, isn't it? They think great thoughts not available to the rest of us. But it is part of Wittgenstein's genius that he sets about to dismantle precisely this very myth, a myth about the nature and epistemological priority of interiority that has been at the heart of western thought since Augustine. It is a myth so compelling and deep-rooted that we continually return to it again and again. Thinking is something we do in our heads. It is a fundamentally private business. Like the origins of a river, thinking can be traced back to some special, private pre-linguistic place, it becomes language inside our heads, then makes its way into the light of day through public utterance.