What I know is that singular common noun 'man' denotes the human being in general sense. In that sense, no article is needed before it, nor should we use the plural form 'men' : "MAN IS MORTAL". (NOT, *"A/The man is mortal" or "Men are mortal".)
But my colleague says that A man or men also denotes the whole class. He says : "A MAN IS MORTAL" or "MEN ARE MORTAL" is also correct. I think "A man" means "A male human being". Could you please clarify whether all the sentences written above in bold letters are the same or not.