The distinction is very clear, where mutual covenants go to the whole of the consideration on both sides, they are mutual conditions, the one precedent to the other. But where they go only to a part, where a breach may be paid for in damages, there the defendant has a remedy on his covenant, and shall not plead it as a condition precedent. If this plea were to be allowed, any one negro not being the property of the plaintiff would bar the action.
Boone v Eyre (1777) 1 H. Bl. 273n.; 2 W. Bl. 1313 at 1314n
Lampleigh v Brathwait  EWHC KB J 17, (1615) Hobart 105:
For wheresoever I build my promise upon a thing done at my request, the execution of the act must pursue the request, for it is like a case of commission for this purpose.
Carlill v Carbolic Smoke Ball Company  1 QB 256, per Bowen LJ:
…if a person chooses to make extravagant promises of this kind he probably does so because it pays him to make them, and, if he has made them, the extravagance of the promises is no reason in law why he should not be bound by them.
Mendelssohn v Normand Ltd  2 All ER 1215, per Lord Denning MR, at p. 1218:
The attendant promised to lock up the car. In other words, he promised to see that the contents were safe. He did not do so. Instead he left the car unlocked… What is the effect of such a promise?… It takes priority over any printed condition. There are many cases in the books when a man has made, by word of mouth, a promise or a representation of fact, on which the other party acts by entering into the contract. In all such cases the man is not allowed to repudiate his representation by reference to a printed condition, see Couchman v Hill, Curtis v Chemical Cleaning and Dyeing Co  1 K.B. 805, C.A. and Harling v Eddy  2 KB 739; nor is he allowed to go back on his promise by reliance on a written clause, see City & Westminster Properties (1934) Ltd v Mudd  2 All ER 733 at p 742;  Ch 129 at p 145, per Harman J. The reason is because the oral promise or representation has a decisive influence on the transaction--it is the very thing which induces the other to contract--and it would be most unjust to allow the maker to go back on it. The printed condition is rejected because it is repugnant to the express oral promise or representation. As Devlin J said in Firestone Tyre & Rubber Co Ltd v Vokins & Co Ltd  1 Lloyd’s Rep 32 at p 39: "It is illusory to say - 'we promise to do a thing, but we are not liable if we do not do it'." To avoid this illusion, the law gives the oral promise priority over the printed clause.
Moschi v Lep Air Services Ltd  2 All ER 393, per Lord Diplock (p.400):
The basic principle which the law of contract seeks to enforce is that a person who makes a promise to another ought to keep his promise. This basic principle is subject to an historical exception that English law does not give the promisee a remedy for the failure by a promisor to perform his promise unless either the promise was made in a particular form, eg under seal, or the promisee in return promises to do something for the promisor which he would not otherwise be obliged to do, ie gives consideration for the promise…
Each promise that a promisor makes to a promisee by entering into a contract with him creates an obligation to perform it owed by the promisor as obligor to the promisee as obligee. If he does not do so voluntarily there are two kinds of remedies which the court can grant to the promisee. It can compel the obligor to pay to the obligee a sum of money to compensate him for the loss that he has sustained as a result of the obligee’s failure to perform his obligation. This is the remedy at common law in damages for breach of contract. But there are some kinds of obligation which the court is able to compel the obligor actually to perform. In some cases, such as obligations to transfer title or possession of property to the obligee or to refrain from doing something to the detriment of the obligee, a remedy to compel performance by a decree of specific performance or by injunction is also available.