To consider metaphysical claims a priori and devoid of empirical content, is a rather commonplace received opinion. This paper attempts an exploration of a contemporary philosophical heresy: it is possible to test metaphysical claims if they play an indispensable role in producing empirical success, i.e. novel predictions. To do so one, firstly, needs to express the metaphysical claims employed in the logico-mathematical language of a scientific theory, i.e. to explicate them. Secondly, they should have an understanding of what it is to test and to verify or to falsify a metaphysical claim. Finally, they also need to consider the philosophical practice of testing a metaphysical claim. These three aspects are introduced in this paper and they are illustrated by means of the metaphysical concept of common cause and the principle of the common cause. 0. Introduction. This paper concerns scientific metaphysics, the branch of metaphysics that deals with specific fields of knowledge and discourse and explores possible domains of existence as determined by scientific theories. In particular, I intend to investigate the testability of metaphysical claims when considered in the context provided by theories of empirical sciences and to explore the