How is semchen (sentient being) defined? Semchen is typically defined as a sentient being.  What does this mean?  What is sentience?

This question about killing bacteria indicates bacteria are not sentient, but I distinctly remember a guru say dust mites are sentient.  Am I imposing my evolution-based viewpoint onto a ladder of sentience?
It depends on the school. Thich Nhat Hanh writes:


  The Diamond Sutra teaches us that it is impossible to distinguish between sentient and non-sentient beings... In Buddhist monasteries, we chant, "Both sentient and non- sentient beings will realize full enlightenment." The First Precept is the practice of protecting all lives, including the lives of minerals.


(Source: http://dharma.ncf.ca/introduction/precepts/precept-1.html)