Draconis's answer is very educational, I had not known that about mediaeval Latin. Many thanks!
But I have to wonder whether there is an answer that Plautus would have understood, namely whether there is an indirect question hidden or understood in this statement.
Q. Estne quod unumquodque ens sub aliquo attributo debeat concipi?
A. Nihil in natura clarius.
Indirect questions usually take the subjunctive in classical Latin, and they are especially hidden in statements of knowledge, perception, or realization. This statement might fit the bill.
Another possibility is that Spinoza is maintaining a certain contemplative distance from the statement, and not necessarily asserting it as a fact yet. (Cf. Woodcock #179)
Without reading the Spinoza with some care, I can't really say whether either of my guesses are relevant. What do you think, Ali Nikzad?