In his commentary to Nicomachean Ethics Aquinas points out that what is most characteristic for reason is the capacity for order, i.e. various kinds of relation of one thing to another (ordo). Now, there are two principal manners in which reason concerns order: (I) it can recognize order — in its theoretical activity, (II) it can establish order and introduce it into some matter — in its practical proceedings. In this connection there are two main types of relation: ones that occur independently from our cognitive acts, and ones that occur as a result of these acts — insofar as it is reason that establishes it. This reason’s activity of establishing order Aquinas calls: ordinare. In my paper I would like to focus on the second type of relation, on the basis of Aquinas’ thought. In particular I would like to study some special type of relations that occur between an end and a means to the end. Therefore, (1) I show in what sense it is only reason that recognizes and establishes relations. (2) I explain the difference between natural relations that can be acknowledged by theoretical reason and relations that can be founded by practical reason, and what is specific for the latter. (3) I distinguish two further kinds of practical relations: technical and moral ones. (4) I study some moral relations, considering especially the ones between a person and his end, a person and his means, and between means and an end. Finally, (5) I show what connection exists between settling these relations and that kind of ordinare which is taking responsibility for action in an act that Aquinas calls imperium (command). Through that I show what relation is there between an agent and his action.