Purpose – The purpose of this paper is to investigate the body-mind problem from a mathematical invariance principle in relation to personality dynamics in the psychological and the biological levels of description. Design/methodology/approach – The relationship between the two mentioned levels of description is provided by two mathematical models: the response model and the bridge model. The response model (an integro-differential equation) is capable to reproduce the personality dynamics as a consequence of a determined stimulus. The invariance principle asserts that the response model can reproduce personality dynamics at the two levels of description. The bridge model (a second order partial differential equation) can be deduced as a consequence of this principle: it provides the co-evolution of the General Factor of Personality (GFP) (mind), the c-fos and DRD3 gens, and the glutamate neurotransmitter (body). Findings – An application case is presented by setting up two experimental designs: a previous pilot AB pseudoexperimental design with one subject and a subsequent ABC experimental design with another subject. The stimulus used is the stimulant drug Methylphenidate (MPD). The response and bridge models are validated with the outcomes of these experiments. Originality/value – The mathematical approach here presented is based on a holistic personality model developed in the last few years: the Unique Trait Personality Theory, which claims for a single personality trait to understand the overall human personality: the GFP.