This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
We have thus far discussed the effect of a contract as between the parties to the contract, and the nature of the rights which it may confer upon third parties. There remains for consideration the question of the liability which a contract imposes upon third persons, and the extent to which they are bound to refrain from interfering with it. Stating the question in another way, a contract right as between the parties thereto is a right in personam. To what extent is it, as to third parties a right in rem which they are bound to respect like other property rights, and to what extent is the right of a person to make contracts in the future a right which others are bound to respect % This is of course a question of tort, and belongs in a discussion of that subject. It is considered here only to complete the statement of the place of the contract in law. The subject must be considered with reference to interference (1) by an individual and (2) by a combination of persons acting in conspiracy. The interference may further be directed against (1) a contract already in existence, or (2) the right of a person to make contracts, though no specific contract has been made. Furthermore the interference may be (1) by persuading a party to the contract to break it; or (2) by making it impossible for him to perform it. This has been said to be " a subject which is likely to be one of the most important and difficult which will confront the courts during the next quarter of a century."1
1 Bohn Mfg. Co. v. Hollis, 54 319; 21 L. R. A. 337; 55 N. W. Minn. 223, 231; 40 Am. St. Rep. 1119.