According to the observation that works of art can in general contain formal properties and nonformal properties, the philosopher Nick Zangwill has delineated three types of formalism as they are encountered at the turn of the 21st century. First, Zangwill identifies "extreme formalists" who think "that all works of art are purely formal works—where a work is purely formal if all its aesthetic properties are formal aesthetic properties," then he defines "anti-formalist" thinkers as those who "think that no works of art have formal aesthetic properties." The third type which Zangwill identifies as representing the transition of the philosophy of aesthetics into the 21st century is that of "moderate formalism", where its principal exponents defend the principle "that all the aesthetic properties of works of art in a select class are formal, and second, that although many works of art outside that class have nonformal aesthetic properties, many of those works also have important formal aesthetic properties that must not be ignored."