I've heard the waffling, reactionary argument that eating plants is no more or less ethical than eating animal flesh because plants, too, are living entities capable of such cognitive feats previously thought only possible by animals. I'm sure all vegetarians and vegans have heard the perennially stale quip, "so what do you have against plants, anyway?"
This is based on a ridiculous misunderstanding and misrepresentation of scientific parlance. In order to facilitate ease of understanding, scientists often use metaphorical, active verbs to describe the functions of nature. Mercury "gallops" around the sun, we "hear echoes" of gamma radiation from the distant universe, plants "forage" for resources.
Yes, plants are alive in the strictest sense and they exhibit chemical responses to external stimuli, but they are not sentient, and sentience - not life - is the key.
Let me humour idiocy far more than it deserves and assume for a moment, hypothetically, that plants were both alive and sentient. Even so, eating them would be unavoidable, and veganism would still be the best way to cause the least amount of harm because you're consuming the plants directly, instead of feeding them to a cow, which is then raised and killed for consumption as well. Raising livestock to be used for food uses approximately ten times the amount of plants needed for a healthy vegan diet, which is one of the reasons why a vegetarian or vegan diet is espoused as more environmentally friendly.
In tomorrow's episode, we'll assume that rocks are sentient and ask what ethical dilemmas are introduced by driving on gravel roads.