פירוש על קידושין 118:2

The Talmud resolves the difficulty by stating that the rules with regard to purity are different for in this realm thoughts can count as action. This is borne out by R. Papa’s statement concerning liquids that make food susceptible to impurity. Food is not susceptible to impurity until it comes into contact with a liquid. The person does not have to put this liquid on the food because the verse is read “if it be put.” However, the verse is written as if it says, “If one put.” So the susceptibility to purity depends on whether the person would want the liquid on the food. If he would, then it is susceptible to impurity. Here we can see that in the realm of impurity, thoughts count like action.