This dissertation investigates the physiological cause of dreams in Greek philosophy and medicine. The thesis is that metabolism is the cause of dreaming for the authors I discuss. In the introduction, I trace this connection between metabolism and dreaming in Heraclitus’ fragments. Chapter 1 treats the Hippocratic treatises On Regimen 1-4. In this chapter, I analyze how the first part of the tetralogy presents metabolism and perception and use these observations to argue that metabolism occurs for this author, in part, when the soul exchanges the sensory stimuli it receives from perception with ingested nourishment. When this happens in sleep, the body and its contents take on the guise of elements from the cosmos, producing dreams. Chapter 2 discusses dreaming and metabolism in Plato’s Timaeus and focuses on the passage about the liver. I explain how the liver functions as a mirror and why – to regulate metabolism. The chapter analyzes how metabolism works in the Timaeus and argues that metabolic failure is another cause of dreaming in this dialogue. I show that dreaming is not a byproduct of metabolism itself, but rather of the soul’s attempt to regulate metabolism as well as the failure of this regulation. Chapter 3 analyzes dreaming and metabolism in Aristotle’s Parva Naturalia. I connect Aristotle’s discussions of dreaming and memory, arguing that the images that appear in sleep are the same ones that form memories, and that these images exist physically in the blood in chain form. The argument continues with an observation I make about Aristotle’s melancholics: their metabolism of food into blood is faster than for regular people, and that since images exist in the blood, their metabolisms allow them to experience more images that together could anticipate future events. I conclude by noting that if metabolism and divination in sleep are related for melancholics, they are for all dreamers. In the Conclusion, I sketch the relation between metabolism and dreaming in Galen. Finally, I discuss current scientific dream theories, underscoring the places in which metabolism and dreaming intersect.