In a teaching given at Instituto Lama Tzongkhapa, Geshe Jampa Gelek said this in response to a student’s question about eating meat:
Regarding the question of eating meat or not, nowadays many people say that the Buddha said that one cannot eat meat in the Vinaya, in Sutra, and some tantras as well. However, if we go back to the Vinaya, which is the basis, the Buddha said that one should not eat meat characterized by three circumstances: seeing, hearing, and thinking. This means that if one sees, hears, or thinks that that meat has been killed on purpose for you or you have a doubt, “I wonder about that” then you should not eat the meat. These are the three circumstances in which one should not eat meat.
The meat that you find in supermarkets, and so forth, was not specifically slaughtered for you. You have not seen, heard, or thought that it was.