We cannot well enjoy two pleasures at once; much less any pleasure almost, whilst pain possesses us.
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“Causes of our judging amiss when we compare present pleasure and pain with future. The cause of our judging amiss, when we compare our present pleasure or pain with future, seems to me to be THE WEAK AND NARROW CONSTITUTION OF OUR MINDS. We cannot well enjoy two pleasures at once; much less any pleasure almost, whilst pain possesses us. The present pleasure, if it be not very languid, and almost none at all, fills our narrow souls, and so takes up the whole mind that it scarce leaves any thought of things absent: or if among our pleasures there are some which are not strong enough to exclude the consideration of things at a distance, yet we have so great an abhorrence of pain, that a little of it extinguishes all our pleasures.” source