The is/ought problem: How can you derive what ought to be from what is?
To think that this problem is a serious one is to overlook the activity of human understanding. When a man understands what a dog is, for instance, being four-legged will usually make its way into the definition. Our experiences with dogs show that dogs have four legs. When we see a three or two-legged dog, we immediately know that, through either a birth defect, disease, external accident, malicious human action, or animal attack, this dog somehow became defective. Implicit in this assumption is that nature acts for an end (final causality). We know that nature does not intend to produce three or two-legged dogs even if we happen to encounter them. In other words, we know that a dog ought to have four legs because that is what-it-is-to-be a dog.
Put another way, the human intellect possess actual knowledge through the act of form, that which makes substances intelligible. Intelligibility is achieved through separation from matter, so our knowledge of form remains even when matter fails to become fully actual in the case of a particular instantiation. This is how we judge two-legged dogs to be imperfect or defective in some way. We do this with everything, this is how human knowing operates.