The thing we need to get out of the way is the idea that externalism has nothing to do with the "boring reason that we are causally affected by our surroundings, so we are likely to notice when those surroundings changed" (421). No one would deny that truism. Of course we are causally affected by our surroundings. Externalism, instead, argues that the "contents of our thoughts are 'fixed' or 'determined' by the context in which they occur" (421). That means that the contents of our thoughts are fixed or determined even if we don't know that this is so. Perhaps it's the case that we never really know that our thoughts are fixed or determined in these ways by our environment. The fact that we don't know is, I suppose, the whole point of the externalist thesis.
We can now ask how they are so determined or fixed without our knowing it.
The incredible thesis of externalism is that
"two indistinguishable thinkers might entertain thoughts with very different contents – thoughts about very different things – if the thinkers are embedded in very different environments" (421).
In what way, then, are the thinkers indistinguishable? This means that their psychological access to their thoughts (and even the thoughts themselves) are indistinguishable even though they have different contents. The fact that they have different contents occurs because they are in different environments; even though they are psychologically indistinguishable. This must mean that the different environments must determine or fix the different contents even if the thinkers are thinking the same things in the same ways. This also means that their minds, when taken apart from their environments, are indistinguishable.
What makes the contents of the thoughts different are the different environments. John Heil argues that this "suggests that there is no way to infer from intrinsic features of a thought to its content: what it is about" (421). We can't analyse their minds, and even what they say about their psychological states, to the real content of those states. The environment, along with internal states, determines and fixes the content. What the states ‘are about’ determine and fix the content of the states. This means that these thoughts can be about something which the thinkers don't effectively know they're about. What they are about, and their environments, fix and determine the contents of such thoughts.
This thesis goes squarely against Cartesian internalism in which all there is to know is what the subject himself knows about his own mind. On that picture, the environment simply doesn't matter – at least not once it has fixed or determined such internal mental states. The Cartesian thinker can break free (as it were) of his environment and still know all there is to know about his own mind and the contents of his own thoughts. This Cartesian vision, however, creates the sceptical dilemma. It means that "agents [are] faced with the problem of 'matching' their thoughts to the world" (422). If minds can exist in splendid isolation from the world, and if agents can know all there is to know, and all they need to know, about their thoughts, then how do they get back to the world and match their thoughts to the world? The radical separation of mind and world has created the sceptical problem of whether or not we do in fact actually correctly match the world when we think about it.
According to externalism, however, there is no such sceptical problem because
"if your thoughts are fixed by the world there can be no question of their 'matching' or failing to match an 'external reality': what your thoughts are about automatically matches the world around you" (422).
If one’s thoughts are fixed by the world in the first place, then the problem of matching or not matching the world ceases to be a real problem. We wouldn't even have these thoughts if they weren't already fixed or determined by the world. We would have no thoughts about the world if the world itself hadn't fixed those thoughts which are about the world. There would be no aboutness if the world had not fixed or determined that aboutness in the first place. The Cartesian wouldn't have thoughts about this or that aspect of the world in the first place if this or that aspect of the world had not already fixed or determined the content of his thoughts about the world (or his thoughts about this or that aspect of the world).
All this lead Putnam to say that "meanings ain’t in the head". Does that mean that meanings are literally in the world? As Heil puts it:
"What we mean by the sentences we utter is partly a matter of how we are situated in the world." (423)
Instead of meanings being parts of the world, so to speak, meaning is a matter of how we are situated in the world. Our place in the world will determine or fix what it is that we mean by our sentences (or words?). What does Heil mean by ‘partly’ a matter of how we are situated in the world? What are the other parts of meaning which we need to know about?
We can think of the Cartesian and externalist pictures in the following ways. The Cartesian thinks that
"words are connected to things… by 'outgoing' chains of significance guided by the agents’ thoughts ('noetic rays') (423)."
The externalist, on the other hand, thinks that words are connected to things by
incoming causal chains.
In the Cartesian picture, the relation is one of mind-to-world. In the externalist picture, the relation is world-to-mind. In the former case, the agent’s thoughts determine the meanings of our words and sentences about the world. In the externalist case, the meanings of our words and sentences are determined and fixed by incoming causal chains.
What are these incoming causal chains? Putnam "emphasises causal connections; Tyler Burge discusses social factors affecting the meaning of what we say" (423). Putnam’s version seems to be a strictly scientific account of meaning (if there can be such a thing). Burge, on the other hand, seems to go back to Wittgenstein’s idea that ‘meaning is use’ – perhaps with an equal emphasis on the causal chains which come from social practices into the minds of agents within those social practices.