Robert Nozick is a heavyweight among philosophers, and Invariances is just what one might expect from him. The book takes up a battery of core metaphysical questions: the nature of truth, objectivity, necessity, consciousness, and ethics. "My own philosophical bent is to open possibilities for consideration, not to close them," he writes. To that end, Invariances asks at least as many questions as it answers. Nonetheless, Nozick tackles his themes rigorously, making this a closely argued and engaging book. Nozick is a political as well as theoretical thinker, and he is among the staunchest proponents of libertarianism. Here he widens his scope to investigate the metaquestions of philosophy and spells out his original conception of objectivity in the world. Nozick, who is Pellegrino University Professor at Harvard, writes with an analytic inclination that can be challenging for lay readers, but his arguments are always intelligent and intriguing. --Eric de Place