Mixed tense: Rene Descartes believed that animals do not have rights.

[BChip]

Is it never permitted at all, even in the following example?

Rene Descartes believed that animals do not have rights.

Thanks.

[Jamie (K)]

It would be permitted in that sentence, because the idea of animals not having rights supposed to be a universal truth.

When something is a scientific fact or is always true, you have a choice of using the present or the past tense in a sentence like that.

[Submerged]

I agree. But then, when you use the present tense here, you're basically expressing your own opinion, not Descartes' one.

[Jamie (K)]

That's not true. You are still expressing Descartes' opinion. How could you possibly be expressing your own opinion if you say "René Descartes believed..."?