A cause that brings something into existence either absolutely or through producing a change in something already in existence. To bring something into existence absolutely is an act that only God can perform and is properly called “creation.” Unlike God, creatures can only bring about changes in things that already exist. 

Instrumental Cause.— An efficient cause that produces its effect by being moved to do so by another efficient cause; an efficient cause that acts by the power of another efficient cause.  This action is formally specified by the higher agent, though presupposing the mediacy of the instrument, such that the higher agency lies within the obediential potency of the instrument.  Thus, air is an instrumental cause of the communication of speech or music. 

Primary Cause/Secondary Cause (also called First Cause/Second Cause).— 1. The relationship that obtains between God and creatures with respect to the effect of any efficient cause. God’s efficient causality is in every case the ultimate source of creaturely efficient causality. 2. Any relationship of efficient causality in which the causality of some agent depends on the causality of another agent.  Secondary causality can be instrumental or non-instrumental.