Dialogues concerning natural religion

The first circumstance which introduces evil, is that contrivance or economy of the animal creation, by which pains, as well as pleasures, are employed to excite all creatures to action, and make them vigilant in the great work of self-preservation. Now pleasure alone, in its various degrees, seems to human understanding sufficient for this purpose. All animals might be constantly in a state of enjoyment: but when urged by any of the necessities of nature, such as thirst, hunger, weariness; instead of pain, they might feel a diminution of pleasure, by which they might be prompted to seek that object which is necessary to their subsistence. Men pursue pleasure as eagerly as they avoid pain; at least they might have been so constituted. It seems, therefore, plainly possible to carry on the business of life without any pain. Why then is any animal ever rendered susceptible of such a sensation? If animals can be free from it an hour, they might enjoy a perpetual exemption from it; and it required as particular a contrivance of their organs to produce that feeling, as to endow them with sight, hearing, or any of the senses. Shall we conjecture, that such a contrivance was necessary, without any appearance of reason? and shall we build on that conjecture as on the most certain truth?

What peculiar privilege has this little agitation of the brain which we call thought, that we must thus make it the model of the wole universe?