Angus Taylor’s Animals & Ethics: An Overview of the Philosophical Debate delivers on its title’s promise: it summarizes the philosophical debate over animals, often phrasing points more clearly than the philosophers did themselves. One of the key figures in this debate is Tom Regan, author of The Case for Animal Rights, and Taylor applauds him for his main contribution to the animal rights debate, “inherent value”:
The key concept in Regan’s philosophy is inherent value. Inherent value is a quality that Regan attributes to every creature that (to put it briefly for the moment) has a life that matters to it. To say that a being has inherent value is to say that it has a value that is independent of any use that it may have for others. Inherent value, then is to be contrasted with instrumental value. To have inherent value, in Regan’s view, is to have the fundamental right never to be treated merely as an instrument, or means, for others. …
The kind of autonomy that Regan says many animals possess is preference autonomy. To have preference autonomy, as he defines it, is to have preferences and the ability to initiate action with a view to satisfying them. In Regan’s view, preference autonomy is the key to having a life that matters to oneself, to being what he calls the subject-of-a-life. Those who are subjects-of-a-life are those who ‘have beliefs and desires; perception, memory, and a sense of the future, including their own future; an emotional life together with feelings of pleasure and pain; preference- and welfare-interests; the ability to initiate action in pursuit of their desires and goals; a psychophysical identity over time; and an individual welfare in the sense that their experiential life fares well or ill for them, logically independently of their being the object of anyone else’s interests (Regan 2004a, p.243). Regan believes that normal mammalian animals of at least a year in age meet this criterion and thus have inherent value and hence moral rights. Birds are probably subjects-of-a-life, and some other creatures may be too (Regan 2003). …
Now, asks Regan, what is it that accounts for our ascription of inherent value to someone, regardless of whether that individual is a genius or a moron, regardless of whether that individual is a morally responsible agent? What relevant similarity can we point to among individuals who have inherent value? Regan answers that what plausibly accounts for our ascription of inherent value to them is the fact that the individuals in question have lives that matter to them, that fare well or ill for them, independently of their usefulness for others…
Further, in Regan’s opinion, this inherent value that we ascribe to persons depends neither on the quality of their experiences nor on whether they are saints or sinners. All who have inherent value have it equally, he says, and it does not matter whether someone is Mother Teresa or an unscrupulous used-car salesperson. (67 – 70)
Taylor does a good job of summing it up, but I thought I’d better consult the original. At the beginning of Chapter 7 of The Case for Animal Rights, Regan unveils his core concept, using slightly more obscure terminology than Taylor:

