The main idea Aristotle tackles in Metaphysics is substance. But concepts such as "potentiality" and "actuality" as well as the nature of God, also contribute to his project of investigating "being qua being". In Metaphysics book Theta, Aristotle uses the concepts of potentiality and actuality to explain the relationship between matter and form in their compound existence. Aristotle held that actuality always precedes potentiality: "one actuality always precedes another in time right back to the actuality of the eternal prime mover". As philosophy professor Terence Irwin points out, in Metaphysics Lambda Aristotle argues that the movements of every astronomical body originate from a separate unmoved mover, but later he also states that, "the universe is unified by a single first unmoved mover". Aristotle's original and innovative ideas about change and the unmoved mover solved known philosophical problems in his time. Aristotle's Metaphysics covers many themes and contains multiple arguments.