In this example, ISTJ, the favourite mental muscle is S. This is deduced by saying: (1) S and T are the two favourite mental muscles (2) The J says that the judgement mental muscle (i.e. T) is in the outer world (3) The I says that the mental muscle in the inner world is the favourite one As T is in the outer world, S is therefore in the inner world, and S is therefore the favourite.