在科学理论中，不充分决定论是指现有的证据未能充分地、圆满地指出某个结论应当获得接纳。以逻辑论解释，即是指存在两个或多于两个命题，使该命题为成立现有证据A的充分条件，但没有任一命题为成立现有证据A的充要条件。此理论亦有被称为亚决定、证据无定理论、不完全决定、证据不足决定理论、证据不充分决定理论、证据无法完全决定理论等等。

举一例：我们知道朋友A买苹果和橙共花了十元，而苹果每个一元，橙每个两元。我们可以肯定朋友A没有买了九个苹果，但凭着以上数据，我们无法确定朋友A各买了苹果和橙多少个，因此，任何对朋友A买了苹果和橙各多少个的结论，按照现有证据都不能充分决定其真确性。

In the philosophy of science, underdetermination refers to situations where the evidence available is insufficient to identify which belief we should hold about that evidence. For example, if all that was known was that exactly $10 was spent on apples and oranges, and that apples cost $1 and oranges $2, then we would know enough to eliminate some possibilities (e.g., 6 oranges could not have been purchased), but we would not have enough evidence to know which specific combination of apples and oranges was purchased. In this example, we would say that belief in what combination was purchased is underdetermined by the available evidence.