Define emptio rei speratae and emptio spei.
Distinguish the two.
Emptio rei speratae (sale of thing expected) is the sale of a thing not yet in existence subject to the condition that the thing will exist and on failure of the condition, the contract becomes in effective and hence, the buyer has no obligation to pay the price.
On the other hand, emptio spei is the sale of the hope itself that the thing will come into existence.
In emptio rei speratae, the future thing is certain as to itself but uncertain as to its quantity and quality. Such sale is subject to the condition that the thing will come into existence (see Art. 1545,par. 2.), whatever its quantity or quality. In emptio spei (like the sale of a sweepstake ticket), it is not certain that the thing itself(winning a prize) will exist, much less its quantity and quality.
In emptio rei speratae, the sale is subject to the condition that the thing should exist, so that if it does not, there will be no contract by reason of the absence of an essential element. On the other hand,in emptio spei produces effect even though the thing does not come into existence because the object of the contract is the hope itself,unless it is a vain hope or expectancy.