Rene Descartes, commonly designated "the father of modern philosophy," held that animals lack all consciousness because

1. they lack language and

2. they lack reason.

Descartes and his followers held that, because they are not conscious, animal experimentation raises no ethical issues at all.

Modern study of animal cognition makes it implausible to hold that animals entirely lack reason, and some studies suggest they can master the rudiments of language. Still, some contemporary philosophers hold that animals may entirely lack consciousness.

The best-known example is Peter Carruthers of the University of Sheffield, England.

In a widely-discussed article called "Brute Experience" (Journal of Philosophy 1989, pp. 258-69), Carruthers argued that although animals clearly experience their environment, their experiences might all be non-conscious, the same way a driver distracted by conversation experiences traffic on the road but can't recall anything about it later. Carruthers proposed that an experience is only conscious if it is available for reflection and on that basis questioned whether animals are ever conscious. Unless we can show that they think about their own experiences, Carruthers argued, we have no evidence that they are conscious at all.

The following articles by Carruthers are available on-line:

• "Animal Subjectivity," in the on-line journal Psyche, details how his views have evolved slightly since "Brute Exper ience."

• "Natural Theories of Consciousness" surveys candidate theories of mind, giving Carruthers' reasons for choosing to define conscious experiences as those which are available to be thought about.

Most contemporary philosophers believe that some non-human animals are conscious, at least in certain ways, and this is certainly a feature of our common-sense world view. But then the question of animals' moral status arises, for if animals can feel pain, suffer, etc., then it would appear that they can be harmed in ways that ought to be taken into consideration from the moral point of view.

The two best-known accounts of animals' moral status are by Peter Singer and Tom Regan, who give very different answers to the questions of why animals count, morally speaking, and how we ought to evaluate our treatment of them.

As philosophers use the terms, Singer's view is an animal welfare perspective and Regan's is an animal rights view. However, the "animal welfare/rights" distinction is commonly employed in a less philosophical and more political sense, and on that popular version of the distinction both Singer and Regan are thought of as animal rightists.