If you have a dog in your house, you may have noticed that when he is asleep he makes little noises and moves or twitches his legs, as though chasing something.
Most dog owners who have had this experience believe this to be a sign that their dog is dreaming. While they cannot say positively that dogs do not dream, scientists will tell you that dogs probably do not dream.
To understand the scientists’ theory, we must remember that men an animals are the products of evolution. This means that over millions of years gradual changes have been taking place. While men and animals are alike in some ways, the mental make-up and senses of animals have evolved in ways different from ours. As a result, animals live in world that are different from ours.
If the senses and the working of the minds of animals are different, we cannot expect their organs and brains to produce what our organs and brains produce. The intelligence and personality of animals are not a “miniature” of the human model.
We cannot know what thoughts animals have. So when we see what we think is a dog dreaming- moving its legs or making noises in its sleep- we cannot assume we are right. The brain cells might just be repeating their messages to the muscles, and the dog might not have any vision of a dream at all.
Those animals that have brains built something like our own may have thoughts like ours, but they will be far simpler.
Source: A book titled More Tell me Why