 Some attempts to give content to the phrase “criterion of identity” can be understood in this framework. Black (1952) argues that purely internal properties of objects do not suffice; his example is a universe with two spheres which share all of their internal properties, i.e., are the same color, weight, composition, etc. There are a wide range of philosophical views about criteria of identity. Brody (1980) takes the two identity and indiscernibility principles as jointly definitive of identity and consequently argues that no further criteria of identity are required. Baker (1997) argues that there are no criteria of identity across time, while Jubien (1996), Merricks (1998) and Zimmerman (1998) articulate reasons to doubt that there are any criteria of identity. Lowe (1989,1997, and 2009) are more recent attempts to clarify the concept