I select a system of multi-site proton-coupled electron transfer (MS-PCET) as the illustrating example in this case (Figure ). MS-PCET leads to a net transfer of hydrogen atom, with the electron and the proton exchange with different sites. In this system, an aryl ketone establishes a hydrogen-bonding pre-equilibrium adduct with a Brønsted acid, where the bound complex undergoes PCET upon excitation of a photoreductant, forming a ketyl radical intermediate. The driving force is jointly determined by the pKa of the acid and the redox potential of the photoreductant. A strong linear correlation between the rate constant and the driving force is obtained no matter which component(s) of the driving force (i.e., acid or photoreductant) is varied. Mechanistically, it strongly suggests that the PCET reduction of aryl ketones is synchronously concerted, that proton and electron transfer have progressed to the same degree at the transition state. However, the correlation constant, Brønsted slope α, is much smaller than the value of α = 0.5 predicted by classical Marcus theory, meaning a much less sensitive ratedriving force dependence. The underlying assumption for the expected α = 0.5 is that the Marcus parabolae of the reactant and the product states have identical shape and differ only in their vertical displacement, which represents the driving force. The Marcus Parabola describes the energy of a collection of atoms in terms of the nuclear position, which is a function of numerous factors such as bonding, conjugation, sterics, solvation, and non-covalent interactions. Each of these affects the parabola's shape. By comparing the reactant and product states in reductive PCET, it is evident that the charge distribution, the hydrogen-bonding framework, and solvation have all changed. Thus, the shapes of the parabolae are very likely to be different, invalidating the assumption of identical-parabolae. Note that while the Marcus parabola representation is practically very useful, it is a very simplistic reduction of the N-dimensional potential energy surface. All models could have the problem of oversimplification. A more physically intuitive explanation is the principle of non-perfect synchronization: the factors determining the energy of the product have developed to different degrees compared to the primary bond changes, influencing the responsiveness of the reaction barrier to the change in driving force. This is also an example of the paradox originating in the reasoning aspect, where subjectivity might come into play. The example discussed above is taken from my previous research. Given my longstanding experience with intrinsic barriers and the principle of non-perfect synchronization, I am less inclined to assume identical Marcus parabolae between the reactant and the product states and so would not find the experimental Brønsted slope surprising. However, I may inadvertently take for granted other types of assumptions with which I am less accustomed.