A door must be either shut or open

You cannot have it shut and open at the same time. You must have one thing or the other, so make up your mind which you want.

See also:

• No man can serve two masters
• You cannot burn the candle at both ends
• You cannot have it both ways
• You cannot have your cake and eat it
• You cannot run with the hare and hunt with the hounds
• You cannot sell the cow and drink the milk
• You cannot serve God and mammon