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.
Illegal contracts are unenforceable not because of any desire On the part of the courts to aid either party thereto, but because public interests require that they be not enforced. If the parties thereto could make them enforceable by the simple device of putting them in writing, using such words as would conceal or omit the illegal objects intended by them to be accomplished, the rules on the subject of illegality would be of but little use. Accordingly, evidence that tends to show that the written contract is illegal,1 as to show that the contract is tainted with usury,2 or is given to compound a felony,3 or that a lease4 is in aid of prostitution, or that a contract is intended to create a monopoly,5 or is in violation of the anti-trust statutes,6 that a contract to lease a railroad is illegal,7 or that a chattel mortgage8 is given to defraud creditors, does not violate the parol evidence rule and is admissible. However, it has been held that it cannot be shown that a note given by a husband to his wife for her release of dower was a part of an oral contract for a collusive divorce.9
13 Knowlton v. Keenan, 146 Mass. 86; 4 Am. St. Rep. 282; 15 N. E. 127.
14 Lawrence County Bank v. Arndt, 69 Ark. 406; 65 S. W. 1052; Southern, etc., Co. v. Ozment, 132 N. C. 839; 44 S. E. 681. See Ch. LVII.
15 Krueger v. Nicola, 205 Pa. St. 38; 54 Atl. 494.
1 McMullen v. Hoffman, 174 U. S. 639; affirming 83 Fed. 372; 45 L. R. A. 410: 28 C. C. A. 178, which reversed 75 Fed. 547; Peed v. McKee, 42 la. 689; 20 Am. Rep. 631; Friend v. Miller, 52 Kan. 139; 39 Am. St. Rep. 340; 34 Pac. 397; Wilhite v. Roberts, 4 Dana (Ky.) 172: Gould V. Leavitt, 92 Me. 416; 43 Atl. 17;
Sherman v. Wilder, 106 Mass. 537; Detroit Salt Co. v. Salt Co., -Mich. -; 96 N. W. 1; Martin v. Clarke, 8 R. I. 389; 5 Am. Rep. 586.
2 Roe v. Kiser, 62 Ark. 92; 54 Am. St. Rep. 288; 34 S. W. 534; Dwelle v. Blackwood, 106 Ga. 486; 32 S. E. 593; Koehler v. Dodge, 31 Neb. 328; 28 Am. St. Rep. 518; 47 N. W. 913; Cotton States Building Co. v. Rawlins (Tex. Civ. App.), 62 S. W. 805.
3 Friend v. Miller, 52 Kan. 139; 39 Am. St. Rep. 340; 34 Pac. 397.
4 Sprague v. Rooney, 104 Mo. 349; 16 S. W. 505; overruling Sprague v. Rooney, 82 Mo. 493, 52 Am. Rep. 383.