found in that for the sale of the barrels of salt described as containing mackerel.  Why is the repugnancy between the two terms,—­first, that the thing sold is the contents of these barrels, and, second, that it is mackerel—­fatal to the existence of a contract?  It is because each of those terms goes to the very root and essence of the contract, 1—­because to compel the buyer to take something answering to one, but not to the other requirement, would be holding him to do a substantially different thing from what he promised, and because a promise to take one and the same thing answering to both requirements is therefore contradictory in a substantial matter.  It has been seen that the law does not go on any merely logical ground, and does not hold that every slight repugnancy will make a contract even voidable.  But, on the other hand, when the repugnancy is between terms which are both essential, it is fatal to the very existence of the contract.  How then do we decide whether a given term is essential?  Surely the best way of finding out is by seeing how the parties have dealt with it.  For want of any expression on their part we may refer to the speech and dealings of every day, 2 and say that, if its absence would make the subject-matter a different thing, its presence is essential to the existence of the agreement.  But the parties may agree that anything, however trifling, shall be essential, as well [331] as that anything, however important, shall not be; and if that essential is part of the contract description of a specific thing which is also identified by reference to the senses, how can there be a contract in its absence any more than if the thing is in popular speech different in kind from its description?  The qualities that make sameness or difference of kind for the purposes of a contract are not determined by Agassiz or Darwin, or by the public at large, but by the will of the parties, which decides that for their purposes the characteristics insisted on are such and such. 11 Now, if this be true, what evidence can there be that a certain requirement is essential, that without it the subject-matter will be different in kind from the description, better than that one party has required and the other given a warranty of its presence?  Yet the contract description of the specific vessel as now in the port of Amsterdam, although held to be an implied warranty, does not seem to have been regarded as making the contract repugnant and void, but only as giving the defendant the option of avoiding it. 2 Even an express warranty of quality in sales does not have this effect, and in England, indeed, it does not allow the purchaser to rescind in case of breach.  On this last point the law of Massachusetts is different.

The explanation has been offered of the English doctrine with regard to sales, that, when the title has passed, the purchaser has already had some benefit from the contract, and therefore cannot wholly replace the seller in statu quo, as must be done when a contract is rescinded. 3 This reasoning [332] seems doubtful, even to show that the contract is not voidable, but has no bearing on the argument that it is void.  For if the contract is void, the title does not pass.