It's hard to say exactly what is chewing on your sage but simply because there are large portions of some of the leaves missing one has to assume it is caterpillars of some sort. Spray with a spinosad based insecticide. It will kill both insects and caterpillars but is harmless to people and pets.
It very well could be slugs or snails if you have them. I should have thought of them but there are not any of them here. There is an organic product named Sluggo Plus that is death on slugs, snails and pill bugs. Just sprinkle it around the plant. It's harmless to people and pets.