慢慢地，他不再那么热衷于仅关注那些在脑内发生的事情，而投向了外在主义阵营——“意义并非存在于脑子里”。他提炼了维特根斯坦的理论，假设了一个语言学范畴的“劳动力”，与亚当史密斯在经济方面所想的类似。那些最能判断一个词的意义的人，在专业知识的基础上也是这样做的。而其他人则只能依赖于日常的使用来判断词义。作为一个哈佛大学的哲学家，他可能无法指出那棵藏在榉树林里的榆树，但他可以自信而有保证地使用这些植物学家才知道到底指的是什么的词。
一个相关的例子，即知识与现实的关系，伴随着另一个思想上的试验而来：一个怀疑论者会怀疑，自己是否仅仅是一个缸中之脑，被人工饲养着，并被一个和真实世界别无二致的虚假世界刺激着而已。有谁可以证否呢？而回答是，我们的大脑并不只是一个接受知觉的机器，而一切意义还要基于别人是怎么思考它的。所以缸中之脑可能存在，但无法说“这只是一个缸中之脑，仅此而已”是有意义的。哲学家们把它称为“认识论的外在主义”，也就是：思维以外的因素是思维能称作会“认识”和“思考”的关键。
Over time he became less keen on thinking of the mind only in terms of what happens inside the brain. Instead he adopted externalism: “meanings just ain’t in the head”. Refining Wittgenstein, he posited a linguistic division of labour, analogous to Adam Smith’s thinking in economics. The people best able to determine a word’s meaning do so on the basis of expertise. Others then rely on their usage. As a Harvard philosopher, he might have no idea how to tell an elm from a beech, but he could confidently use the words with the assurance that botanists knew exactly what they meant.
A related example of the tension between knowledge and reality came with another thought experiment: a sceptic might wonder whether she were no more than a brain in a vat, artificially nourished, and stimulated with a bogus but utterly convincing version of the real world. How could one prove that this is not so? The answer is that our brains are more than just perception machines, and meaning depends on what other people think too. So a brain in a vat might exist, but it could not meaningfully say that it was merely a brain in a vat. Philosophers would call that epistemological externalism: factors outside the mind are crucial to what it can be said to “know” and “think”.
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