פירוש על קידושין 118:1

The mishnah quoted here is about purity laws. A vessel becomes susceptible to impurity when one thinks that one has finished it. For instance, if I’m making a dish and I intend to glaze it, it is not susceptible to impurity until I do so. But if I think to use it unglazed, it is susceptible to impurity before it is glazed. In order for it to become not susceptible to impurity, one would have to do something to it, like break it.
The mishnah then summarizes the power of acts and intention. An act is powerful enough to annul an intention or another act, but intention is not powerful enough to annul either speech or action.
This is a difficulty against R. Yohanan who holds that speech should be able to cancel other speech.