In theory, despite the fact that one party retained and paid for the services of an expert witness, expert witnesses are supposed to testify impartially in the sphere of their expertise. Thus, one can call an expert witness even if one disagrees with the testimony of the expert. Rule 801(d)(2)(C) requires that the declarant be an agent of the party-opponent against whom the admission is offered, and this precludes the admission of the prior testimony of an expert witness where, as normally will be the case, the expert has not agreed to be subject to the client's control in giving his or her testimony. See Sabel v. Mead Johnson & Co., 737 F. Supp. 135, 138 (D. Mass 1990). Since an expert witness is not subject to the control of the party opponent with respect to consultation and testimony he or she is hired to give, the expert witness cannot be deemed an agent. See Restatement (Second) of Agency § 1 cmt. a (1958) ('The relation of agency is created as the result of conduct by two parties manifesting that one of them is willing for the other to act for him subject to his control, and that ...