Derek: Try me. I would honestly like to know what it is that makes it ok to eat cows but not humans. But if you aren't articulate enough to explain it then don't bother.
How can I explain this to Derek?
Cows are incapable of giving consent to the rights and responsibilities of society. They therefore do not get the same rights as those who do.
Longer answer:
Firstly, no, there are some humans who cannot give consent. Typically, they already have or might in the future (disabled people and babies respectively). You could build an argument about that.
If he queries why consent is important, you can similarly ask why the ability to suffer is important, that probably being the basis of his views. Both are arbitrarily chosen. He can use the argument to query your opinions, but his opinions are just as vulnerable. The conclusion therefore is certainly not to avoid eating cows.
Next up, he'll ask why rights and responsibilities go together: why a cow must consent to the responsibilities of society before it can have rights. At this stage we have to admit that it could be granted legal protections by society without consenting to them, but we will also point out that there is no over-riding moral justification for such protections.
Rights and responsibilities go together because they are the inverse of one another. The right not to be murdered is simply everyone else having a responsibility not to murder you.
At this stage we are getting into quite fundamental ideas about whether rights are fundamental and universal, which they quite clearly are not.
You could say that it is incredible that he can imagine an obligation governing a person who has not consented to it. That's a perfect description of a fundamental right: if a cow has a fundamental right not to be eaten, then this is the same as an obligation on me not to eat it, despite me never agreeing to such a thing.
You could also ask him whether predators in the wild are criminals. He will probably come up with some foolish notion that 'they know no better'. We can ignore his assumption of superiority, and bring up an interesting point of established law:
ignorance of the law is not an excuse for breaking it.
We can also wonder why being able to think suddenly gives us obligations, without even consenting.
Let us quote a wise old man (Albus Dumbledore): 'it is the choices we make that truly determine who we are'.
Thinking is not a choice. Consent is the choice. Consent is the action. If I am a criminal simply by virtue of my ability to think then I have been condemned for something that I cannot control.
That is the morality of fundamental rights. They grant rights on the basis of characteristics that are beyond the control of the being that is being given those rights, and are also beyond the control of the beings that must respect those rights. Fundamental rights therefore impose responsibilities, based solely on the belief of the person ascribing those fundamental rights. If we're going to choose arbitrary characteristics, we could just as easily choose humanity as the characteristic that determines whether a being gets rights or not.
On the other hand, if we believe that such an imposition is impossible to justify, the only way that responsibilities can govern a being is if that being has consented to them. This makes much more sense, since the consequences (being wrong or right in a moral sense) are now dependent on our actions, not on arbitrary characteristics such as being able to think, and we typically view moral judgement as being a way of judging actions.