Commentary for Kiddushin 132:2

Rava will quote a long baraita that will eventually prove his point. The baraita refers to a reservoir (a small one it seems) that had been used as a mikveh but then turned out not to have enough water in it. Can we assume that when people or things which had been immersed in it were immersed, the mikveh was already lacking? R. Tarfon rules leniently—that which was immersed in it is assumed to be pure. R. Akiva rules stringently—everything is impure.

R. Tarfon notes that the assumption is that the mikveh is full. It remains under the presumption of being full until it can be proved lacking. In contrast, R. Akiva looks at the person—the person is assumed impure until it can be proven that he is pure.