Final answer:
The fundamental assertion of positive psychology therapy is that good mental health requires more than the absence of mental illness.
Step-by-step explanation:
The fundamental assertion of positive psychology therapy can be summarized as: Good mental health requires more than the absence of mental illness. Positive psychology focuses on what is good for a person and aims to promote well-being by emphasizing intrinsic goods that contribute to a flourishing life. It recognizes that mental well-being is not solely determined by external circumstances and that only focusing on the absence of mental illness is not sufficient for good mental health.