The philosophical doctrine known as physicalism attempts to explain the world without any reference to abstract entities such as minds, thoughts, or choices. Physicalist explanations can refer only to physical phenomena, so physicalists deny the existence of “downward causation”, or the ability of thoughts to exert causal influence on the brain or body. Yet we have all experienced the tensions in our bodies that we call “the stress response”, and it is a familiar psychological fact that these sensations often co-occur with spirals of negative thought patterns. The physicalist may argue that this co-occurrence is a mere correlation, which is susceptible to a purely physical explanation. He might argue, for instance, that all our negative thoughts are mere epiphenomena. Or he may argue that our negative thoughts actually result from—rather than cause—the physical stress response. And although it may appear that such physicalist accounts explain the correlation between tensions in our bodies and negative thought patterns, no physicalist account can possibly explain why the contents of particular thought patterns have a negative valence for the thinker. And because one cannot describe the correlation itself without referring to the negativity of the thought patterns that co-occur with the stress response, a failure to explain the negativity of the thought pattern cannot satisfactorily explain the correlation. That is, it is only by admitting downward causation that we can explain both the negative valence of the thought patterns and the co-occurrence of these thought patterns with the bodily sensations that characterize the stress response.