How many thoughts can you think?

In ordinary computer programs, the relationship between data in a machine and the concepts it represents is defined arbitrarily by the programmer. It is argued here that the Strong AI hypothesis suggests that no such arbitrariness is possible in the relationship between brain states and mental experiences, and that this may place surprising limitations on the possible variety of mental experiences. Possible psychology experiments are sketched which aim to falsify the Strong AI hypothesis by indicating that these limits can be exceeded. It is concluded that although such experiments might be valuable, they are unlikely to succeed in this aim.