Among the perennial questions of Western philosophical tradition is one about the existence of the 'external' world. In its most basic form, the question asks, Do things exist apart from our thoughts about things? Is it true, in other words, that to be is also to be perceived?--to borrow a phrase from the 18th century Anglo-Irish philosopher Bishop George Berkeley. Does the tree exist because you think about it, or does it exist prior to your thinking about it?
But the answer to the question is not as obvious as it first appears. The longer you think about the question, in fact, the more obscure the it becomes.
We have good reasons for thinking things exist apart from our thoughts about them. To begin with, we fall asleep at night and wake up to find the world much the same as we left it. We travel familiar routes to home, school, or work, navigating by means of familiar landmarks. The continued presence of objects in our physical environment provides a very strong reason to think they exist apart from our conscious perception of them.
As far as a naive faith in the external world goes, philosophers seem the worst of the bunch. They can always be found talking about Kant's view of this or Heideigger's view of that, as if Kant and Heidegger, and their views of this or that, were out there waiting to be looked at, thought about, and discussed at great length. Which, of course, they are--recorded for us in books.
We are very comfortable with the thought that an external world exists apart from our thought about that world. It helps us make sense of learning, discovery, and being in error. Something 'external' to our thinking provides a standard against which to measure the truth of our thought. Our thought runs up against it, tries to comprehend it, arrives at a provisional understanding, makes a decision as to its adequacy, and so on. We presume the existence of an external world whenever we communicate our thoughts with others. At least, those of us do who have not yet figured out how to communicate directly, one mind to another. Not only do we make use of the external world as a medium of communication, much of our communication has to do with calling others attention to consider some object found there.
Not everyone is happy with the language of an external world, nor the implied idea that the world is one thing and thought about the world another. (The aforementioned Kant is a good example.) The philosopher Daniel Dennett has coined the term 'Cartesian theatre' to capture how strange the idea of thinking about the world as external to ourselves. The most obvious reason for why the idea just doesn't measure up, of course, is that we find ourselves in the external world: our bodies. We are, in some very real sense, our bodies. As our bodies move, so we grow. As our bodies grow, so we grow. Where our eyes look, our conscious attention seems to follows--or does it lead? Dennett enjoys mocking persons who think of themselves as looking at themselves (their body) from an undefined location (their mind). The mind is not the brain, after all. The brain is something that can be seen, picked at, pulled apart, sliced into sections. The same cannot be done to the mind, per the definition of mind. But if it can't be observed and studied, it seems legitimate to wonder whether the thing exists at all.
I haven't much time for Dennett's endless refusal to say anything positive about this thing I call myself, though I find his line of questioning to be a helpful foil. Thomas Nagel has it exactly right when he says that Dennett merely redefines consciousness as an external property, ignoring the essential problem, which is the subjective first-person perspective that each of us occupies, and no one else does for us. Indeed, it's the individual's first-person perspective (which, if re-ified, is called an immaterial soul, the life of the rational animal) makes the external world a problem in the first place.
The individual first-person perspective throws a monkey wrench into any abstract formulation--whether it's Berkeley's to be is to be perceived or Dennett's critique of the 'Cartesian theatre'. Certainly the logic of these positions can be tried and tested; but logical analysis aims at universal applicability, which is precisely not a first-person perspective. If the world exists only because I perceive it, the rest of you have a real problem. Likewise, if a first-person perspective is nothing, or at least nothing worth thinking about, then we, each individually, all have a real problem.
Bishop Berkeley had an answer. To be can still be to be perceived, even if no human being is perceiving every single object in external world all the time claimed Berkeley, because the being we call God perceives everything, which allows them also to exist apart from partial human perspectives. That not a solution open to Dennett, at least not one he thinks is open to him. So he runs away from the first-person perspective; and, we might say, trips over the elephant in the room--himself.
The idea of a world external to ourselves, it seems to me, helps us all make sense of our individuality. It allows me to say your perspective on things may differ from my perspective on things by creating a buffer zone between the part of me only I have access to and the part of me the rest of the world gets to see. You are external to me. We can talk things out, but we won't necessarily come to an agreement, or even an understanding. And that is okay.