The same may be said when several proofs establish a single conclusion: they yield reinforced knowledge, or multiple instances of knowledge of the same kind. Hence, together they provide a clarity and strength that no single proof provides.
This falls under the general principle that whenever a causal factor—whether an agent acting by will and choice or by nature, or a motive prompting and impelling action—has a partner in its action and influence, that partner assists and supports it and prevents it from independently and exclusively producing the result. It follows that each needs the other and is not self-sufficient in its action. Joint participation thus entails dependence and precludes self-sufficiency, for two agents participating in an action assist one another in performing it.
Neither can, on its own, do what it does together with the other, provided that it remains unchanged whether acting jointly or alone. For if an agent produces something on its own, and we suppose that it remains unchanged and joins another like itself, their joint effect cannot be the same as the effect each produces alone. For if an effect owes its existence solely to its agent, and the agent produces an effect when acting alone, then, when the two combine, their joint effect must be greater in quantity or magnitude than the effect of either alone. Otherwise, the greater would be no different from the lesser.
It is different, however, when the agent undergoes a change. A person, for example, may lift a piece of timber or prepare food together with another person, and then do the same alone. This necessarily involves some change on his part in his will, movement, implements, or the like. Otherwise, if his condition remains the same in both cases, the effects produced when acting alone and when acting jointly cannot be equal.