Well, that definition, like most dictionary definitions of complex philosophical or scientific concepts, is circular or too vague. The quality of being sentient

Well, that definition, like most dictionary definitions of complex philosophical or scientific concepts, is circular or too vague. The quality of being sentient is obviously circular, Consciousness is even more vague a concept than sentience, and Susceptibility to sensation is either circular (if we seek to define sensation narrowly) or too vague (if we define sensation broadly - after all, rocks are susceptible to sensation in some sense, as they immediately move when you kick them). This is not a critique of the OED: dictionaries are meant to explain to humans what other humans mean when they use a word, not give an exhaustive understanding of its deeper meanings and implications (encyclopedias aim for that). When actually trying to rigorously define the word "sentience" we currently have settled on a very basic but measurable criterion: does the living thing we are discussing react to stimuli in its environment, and does it do so in a manner which suggests some kind of memory or other information processing power? Conversely, consciousness has no similar even slightly rigorous definition, but it is still used as a "higher level" of sentience, sentience + some other stuff that bring you closer to how humans experience the world. Sapience seems to fit somewhere in between.