Full context:   The philosophy of mind studies the nature of mental phenomena and how they are related to the physical world. It aims to understand different types of conscious and unconscious mental states, like beliefs, desires, intentions, feelings, sensations, and free will. An influential intuition in the philosophy of mind is that there is a distinction between the inner experience of objects and their existence in the external world. The mind-body problem is the problem of explaining how these two types of thing—mind and matter—are related. The main traditional responses are materialism, which assumes that matter is more fundamental; idealism, which assumes that mind is more fundamental; and dualism, which assumes that mind and matter are distinct types of entities. In contemporary philosophy, another common view is functionalism, which understands mental states in terms of the functional or causal roles they play. The mind-body problem is closely related to the hard problem of consciousness, which asks how the physical brain can produce qualitatively subjective experiences.