What is the test for evidence of mental illness in a murder case?

[11 Cal.App.4th 300] "Section 28, subdivision (a) provides in pertinent part that evidence of mental illness 'shall not be admitted to show or negate the capacity to form any mental state,' but is 'admissible solely on the issue of whether or not the accused actually formed a required specific intent, premeditated, deliberated, or harbored malice aforethought, when a specific intent crime is charged.' (Italics added.)" (People v. Saille (1991) 54 Cal.3d 1103, 1111-1112, 2 Cal.Rptr.2d 364, 820 P.2d 588.)