One is a sentient being (sattva) in virtue of not recognizing Buddha-nature. Such a life is one of confusion (samsara) desiring what is not Buddha-nature. While potentially all sentient beings can recognize their Buddha-nature their delusions bind them to a wayward life of suffering. Even those sentient beings who are desirous of realizing their Buddha-nature are limited by their delusions. Such a burden amounts to mistaking what is not Buddha-nature for Buddha-nature.
From the beginning, sentient beings have not recognized their Buddha-nature. As a result of this non-recognition or avidya, samsara is generated. There is no escaping the fact that sentient beings have never known any other condition except samsara. Still, it is almost impossible for us to accept that Buddha-nature is the fundamental nature of existence; more than our present way of life which is fictional. It goes against all of our beliefs.
So what is Buddha-nature? As the Tibetan Lamas tell us, it is utterly open. We can think of it as a limitless, pure, open field or medium. In addition, it is a dynamic—yet totally invisible and fundamentally empty. It is luminous (prabhâsa) or the same, radiant. Some call it clear light. To experience it directly is to understand what compassion or karuna really means; seeing it mystically at work in the world trying to awaken the world. By realizing it one no longer remains entangled with phenomena since phenomena do not actually exist. Only this pristine nature or Mind-nature exists.