  The problem with this thought is that not all natural properties are directly observable. Some kinds of natural properties are highly complex, and knowable only through the functional role they occupy. Consider, for instance, the property of being healthy. Being healthy isn’t like being red; there’s no way that healthy people look. Of course, there may be some characteristic visual signs of healthiness—rosy cheeks, a spring in one’s step—but these visual signs are neither necessary nor sufficient for healthiness. These directly observable properties are only indications of healthiness. Healthiness is a complex natural property, wholly constituted by an organism’s body being in the “proper” configuration. Healthiness has a robust causal profile. There are many things that can cause or impede health by their presence or absence: food, water, disease, etc. And there are many things that will result from health in typical circumstances: energy, long life, etc. Rosy cheeks and a spring in one’s step are indications of healthiness because these are properties that are—typically—caused by health. Our awareness of the causal profile of healthiness thus gives us a way of figuring out which things have or do not have the complex property of healthiness.