A CHIMPANZEE talks with humans using symbols. Squirrels outfox a gun-wielding homeowner tired of hearing them in his attic. Elephants pause as if in mourning when they stumble upon pachyderm bones in the wild. Yes, many experts say, animals do think. The question is: What goes on in their minds? ''Is it instinct or do they think like us, in steps? That's the whole trouble, you see, we don't know where instinct ends,'' said David Pears, an Oxford philosopher. ''If you see a red light, you stop. You don't say to yourself, 'OK, red means stop.' ''We have to define the level of thinking. Just because a species thinks differently from us doesn't mean it's less cognitive.'' For centuries, scientists followed the lead of philosopher Rene Descartes, who said most animals had little self-awareness, much less cognitive ability. But recent experiments with animals such as Kanzi, a chimp that communicates through symbols on a computer, have convinced many scientists animals can think. But beyond that, is their thinking limited to imitating others? Or do they work out problems? What do they think about? Can they lie? Don't expect a consensus from the animal intelligence experts who gathered in Atlanta recently. After all, said Harvard neurobiologist Terrence Deacon, doctors are only now learning how the human brain works. Because we cannot communicate with animals effectively, learning what's in their minds will be even more difficult. ''There's a different cognitive style - it's not just that we're smarter,'' he said. Researchers may never prove whether animals lie, said Georgia Tech's Jack Martin, who fell short in an experiment in which one monkey would tell a rival the coast was clear when a stuffed python was placed near them. It couldn't be determined if the monkey was lying or knew the snake wasn't real. Scientists may also mistake routine reactions or imitation for advanced thought, Deacon warned. But chimpanzees at Georgia State University's Language Research Centre communicate using a computer that takes away that guesswork, said researcher Sue Savage-Rumbaugh. Kanzi types on a keyboard with symbols. The order of the symbols resemble sentences. And he types original sentences - eliminating criticism that he's just copying people, Savage-Rumbaugh said. ''He types 'ball-group room,' something I have never said to him,'' she said. ''Why else would he say that unless he wants a ball he knows is in the group room instead of the ball in front of him?'' In the wild may be the best place to prove animals think on their own rather than just under human influence, contends Pears. Elephants obviously ponder something when they pause and make mournful sounds over the remains of fellow pachyderms, he said.