According to the Crash Course Philosophy video today, George Berkeley summarized his empirical philosophy with the phrase "esse est percipi", to be is to be perceived. However, it feels somewhat incorrect to use infinitives this way. "Cogito ergo sum", I think, therefore I am, uses present singular. If "seeing is believing" were to be translated, it feels like it should use gerunds, although it could be converted to "to see is to believe", which sounds more awkward. So then, why infinitives in this case?
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The expression esse est percipi is grammatical. Notice that the gerund does not have a nominative form at all. If you want the corresponding nominative (or accusative when there is no preposition), you need to use infinitive.
The grammatical structure is the same as in giraffa est alta ("the giraffe is tall"). You are simply saying that something is something, and it does not really matter grammatically what these things are. The predicate is est and the subject can be esse just as well as giraffa.
If you want to say "seeing is believing", infinitives are a good choice again. You can say the same thing in English as "to see is to believe" — there might be a slight difference in nuance but the meaning is the same. I would suggest videre est credere. You can't really phrase this as concisely with any other structure.
On the other hand, cogito ergo sum is a very different kind of sentence. It has two clauses and the subject in both of them is the implicit ego.
Cogito ergo sum does not mean "seeing is believing". It in fact means "I think therefore I am." Decartes used it as a statement of epistemology: If he can think, if he can conjure up rational process, it follows that he must exist. It establishes the I.
In Latin, there is no nominative of the gerund. What that means is that you'll never see a gerund as the subject of the sentence. Instead, the infinitive is used. A case can actually be made that the infinitive functions as the nominative of the gerund. (It can't actually be the nominative, but essentially they're equivalent.)
Therefore, indeed esse est percipi is fully grammatical.