Within metaphysics itself there are a wide range of differing philosophical theories. Idealism, for example, is the belief that reality is mentally constructed or otherwise immaterial while realism holds that reality, or at least some part of it, exists independently of the mind. Subjective idealism describes objects as no more than collections or "bundles" of sense data in the perceiver. The 18th-century philosopher George Berkeley contended that existence is fundamentally tied to perception with the phrase Esse est aut percipi aut percipere or "To be is to be perceived or to perceive