Sentience has never been well defined. One basic definition of sentience is whether the lifeform in question is 'self aware', but a true definition of sentience itself is rather subjective. If you've ever read Chobits, the plot there was that these personal computers called 'persocoms', which were human in appearance and could do everything a personal computer would do, but were not actually sentient but instead had very complex software and hardware. The idea that was bandied about in the series was that these special computers called 'chobits' existed, which could extend themselves beyond their programming; effectively, they could feel and act in ways which were not simply by design, effectively sentient. The main persocom in the series, Chi, was thought to be one, and throughout the series the main human protagonist, Hideki, is conflicted about how he feels for her, knowing that she's only a computer, despite the fact that her 'programming' closely mimics real human interactions. Effectively, whether actually 'loving' something like that, or treating it as equal to a human being, would be wrong. It was a rather nifty series, actually...