 How does the BTT address the apparent problem of “brain interventions,” such as LSD, microelectrodes, or brain tumors? The answer is multi-faceted. The quickest method for taking much of the sting out of these cases is to note that they generally do not arise for most individuals. The Best Test Theory relies on personal biographies in which only actual instances of kinds triggering mental representations are used to specify causal efficiency. The counterfactual truth that, were a stimulating microelectrode to be applied to, say, a particular neuron, it would perfectly reliably produce an “X” token simply does not matter for the theory. So, for all those individuals who do not take LSD, do not have microelectrodes inserted in their brains, do not have brain tumors, etc., these sorts of counterfactual possibilities are irrelevant. A second line of defense against “brain interventions” appeals to the limitation to natural kinds. The BTT might set aside microelectrodes, since they do not constitute a natural kind. Maybe brain tumors are; maybe not. Unfortunately, however, LSD is a very strong candidate for a chemical natural kind. Still the BTT is not without a third line of defense for handling these cases. One might suppose that LSD and brain tumors act on the brain in a rather diffuse manner. Sometimes a dose of LSD triggers “Xi”, another time it triggers “Xj”, and another time it triggers “Xk”. One might then propose that, if one counts all these episodes with LSD, none of these will act often enough on, say, “Xi” to get it to mean LSD, rather than, say, dog. This is the sort of strategy that Rupert invokes to keep mental symbols from meaning omnipresent, but non-specific causes such as the heart. The heart might causally contribute to “X1”, but it also contributes to so many other “Xi”s, that the heart will turn out not to be the most efficient cause of “X1”.