"see" is a stative verb in its basic meaning and doesn't usually take the -ing form in English.
We say, "I know that, I love you, I see three ships, I don't think so, I hope not" etc. rather than "I am knowing that, I am loving you, I am seeing three ships, I am not thinking so, I am hoping not".
There is an alternate meaning of "see" (meaning "to go on dates with regularly") which does form the -ing form.
So "János sees Pál" means that János opens his eyes and notices Pál with them, but "János is seeing Pál" means that the two are going out together frequently. They don't mean the same thing.