Simplicity is not inimical to patentability. In re Oetiker, 977 F.2d 1443, 24 U.S.P.Q.2d 1443, citing Goodyear Tire & Rubber Co. v. Ray-O-Vac Co., 321 U.S. 275, 279, 64 S.Ct. 593, 594, 88 L.Ed. 721 (1944) (simplicity of itself does not negate invention); Panduit Corp. v. Dennison Mfg. Co., 810 F.2d 1561, 1572, 1 USPQ2d 1593, 1600 (Fed.Cir.) (the patent system is not foreclosed to those who make simple inventions), cert. denied, 481 U.S. 1052, 107 S.Ct. 2187, 95 L.Ed.2d 843 (1987). Roberts v. Sears, Roebuck, Co., 723 F.2d 1324, 1334, 221 U.S.P.Q. 504 (7th Cir. 1983)(“[I]ntuitive analysis distorted by the invention’s simplicity and retrospective self-evidence must be avoided. Simplicity is not to be equated with obviousness.”); Chore-Time Equip., Inc. v. Cumberland Corp., 713 F.2d 774, 218 U.S.P.Q. 673 (Fed.Cir. 1983)(“That is not to say that simplicity of the invention, or its ready understandability by a judge, constitute evidence of obviousness.”) 

In DatCard Sys., Inc. v. PacsGear, Inc., No. 8:10-cv-01288-MRP, 2013 BL 440221 (C.D.Cal. Mar. 12, 2013), the district court said:

To be sure, the Court’s characterization of the patents-in-suit as “simple” is hardly a clue regarding its opinion regarding their validity under, for example, Section 103 . Patent law lacks a doctrine of simplicity. To be sure, as a matter of logic, technically simple patents are perhaps more vulnerable to obviousness-based invalidity than technically complex patents. But they are not obvious because they are technically simple.

(emphasis added)

But, see, Skee-Trainer, Inc. v. Garelick Mfg. Co., 361 F.2d 895, 150 U.S.P.Q. 7 (8th Cir. 1966)(“[A]lthough the simplicity of a mechanical innovation will not automatically render it unpatentable, simplicity is itself strong evidence that ‘invention’ is lacking.”)