"Showing how the initial researchers may have been cooperatively involved in the two abortions helps to make clear how vaccine producers cannot be understood to be cooperatively involved. In the context of vaccine production, the principle of cooperation is (at most) applicable to evaluating the relationship between the people who performed the abortions and the people who initially obtained the tissue. This is because the principle of cooperation applies only when there is some shared, cooperative action. The two fundamental kinds of cooperation are called material and formal. At the simplest level of analysis, material cooperation occurs when someone contributes something that enables another person to perform some action. For example, a nurse assisting at an operation and someone who lends another person money cooperate materially in the operation and in the use of the money. Theologians distinguish many degrees and kinds of material cooperation, some forms of which are so closely connected to the principal action that the cooperator shares in the moral responsibility of that action. Nevertheless, this is not always the case. For example, an employer pays employees and thereby cooperates in their financial activities, but the employer remains normally free of moral responsibility for how employees use their money."