If you mean "sometimes a particular animal can be on the cusp between life and death and it's therefore hard to tell which term applies", you may have a point,

If you mean "sometimes a particular animal can be on the cusp between life and death and it's therefore hard to tell which term applies", you may have a point, although for most animals (including people) observation tells us the cusp is very small. If you mean "life" or "death" are merely mental categories that don't have existence in reality, then saying "this is alive" or "this is dead", or "this is close to life" or "this is close to death", would all be meaningless. We can't predicate a purely mental category of something and then claim that the statement has a meaning outside our minds. Nor can we say our mental categories are approximations of anything real unless we categorize based on something that really exists in the first place.