In addition to the materialism of the empiricists, under the same aegis of reason, rationalism produced systems that were diametrically opposed, now called idealism, which denied the reality of matter in favor of the reality of mind. By a 20th-century classification, the idealists (Kant, Hegel and others), are considered the beginning of continental philosophy, while the empiricists are the beginning, or the immediate predecessors, of analytical philosophy. Some philosophers deny that the concept of "being" has any meaning at all, since we only define an object's existence by its relation to other objects, and actions it undertakes. The term "I am" has no meaning by itself; it must have an action or relation appended to it. This in turn has led to the thought that "being" and nothingness are closely related, developed in existential philosophy. Existentialist philosophers such as Sartre, as well as continental philosophers such as Hegel and Heidegger have also written extensively on the concept of being. Hegel distinguishes between the being of objects ("being in itself") and the being of people ("Geist)". Hegel, however, in his "Science of logic", did not think there was much hope for delineating a "meaning" of being, because being stripped of all predicates is simply nothing. Heidegger, in his main work "Being and Time", in his quest to re-pose the original pre-Socratic question of Being, wondered at how to meaningfully ask the question of the meaning of being, since it is both the greatest, as it includes everything that is, and the least, since no particular thing can be said of it. He distinguishes between different modes of beings: a privative mode is present-at-hand, whereas beings in a fuller sense are described as ready-to-hand. The one who asks the question of Being is described as "Dasein" ("there/here-being") or being-in-the-world. Sartre, popularly understood as misreading Heidegger (an understanding supported by Heidegger's essay "Letter on Humanism", which responds to Sartre's famous address, "Existentialism is a Humanism"), employs modes of being in an attempt to ground his concept of freedom ontologically by distinguishing between "being-in-itself" (things), and "being-for-itself" (man). Being is also understood as one's "state of being," and hence its common meaning is in the context of human (personal) experience, with aspects that involve expressions and manifestations coming from an innate "being", or personal character.