Now, how do we settle whether such a condition exists? It is easy to err by seeking too eagerly for simplicity, and by striving too hard to reduce all cases to artificial presumptions, which are less obvious than the decisions which they are supposed to explain. The foundation of the whole matter is, after all, good sense, as the courts have often said. The law means to carry out the intention of the parties, and, so far as they have not provided [335] for the event which has happened, it has to say what they naturally would have intended if their minds had been turned to the point. It will be found that decisions based on the direct implications of the language used, and others based upon a remoter inference of what the parties must have meant, or would have said if they had spoken, shade into each other by imperceptible degrees. Mr. Langdell has called attention to a very important principle, and one which, no doubt, throws light on many decisions. /1/ This is, that, where you have a bilateral contract, while the consideration of each promise is the counter promise, yet prima facie the payment for performance of one is performance of the other. The performance of the other party is what each means to have in return for his own. If A promises a barrel of flour to B, and B promises him ten dollars for it, A means to have the ten dollars for his flour, and B means to have the flour for his ten dollars. If no time is set for either act, neither can call on the other to perform without being ready at the same time himself. But this principle of equivalency is not the only principle to be drawn even from the form of contracts, without considering their subject-matter, and of course it is not offered as such in Mr. Langdell's work.