What is the strongest rationale for combining references?
The strongest rationale for combining references is a recognition, expressly or impliedly in the prior art or drawn from a convincing line of reasoning based on established scientific principles or legal precedent, that some advantage or expected beneficial result would have been produced by their combination.
The answer to this question can be found in chapter 2100 of the MPEP. This chapter covers Patentability.
The answer is from the 9th Edition, Revision 07.2015. Depending on future changes to the MPEP, the question and answer may or may not be applicable in later Editions or revisions.
This question and answer comes from section 2144 of the MPEP. The following is a brief summary of section 2144.
2144 Supporting a Rejection Under 35 U.S.C. 103
This section discusses supporting a rejection under 35 U.S.C. 103. Office personnel may invoke legal precedent as a source of supporting rationale when warranted and appropriately supported.
Rationale may be in a reference, or reasoned from common knowledge in the art, scientific principles, art-recognized equivalents, or legal precedent. The expectation of some advantage is the strongest rationale for combining references.
Legal precedent can provide the rationale supporting obviousness only if the facts in the case are sufficiently similar to those in the application. In addition, a rationale different from the applicant’s is permissible.