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. /1/1 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from the contract, and therefore cannot wholly replace the seller in statu quo, as must be done when a contract is rescinded.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. It might be said that there is no repugnancy in the charterer's promise, because he only promises to load a certain ship, and that the words "now in the port of Amsterdam" are merely matter of history when the time for loading comes, and no part of the description of the vessel which he promised to load. But the moment those words are decided to be essential they become part of the description, and the promise is to load a certain vessel which is named the Martaban, and which was in the port of Amsterdam at the date of the contract. So interpreted, it is repugnant.