*286Concurring Opinion by
Mr. Justice Roberts:I join fully in the Court’s holding that appellee has adequately established his contractual right to the disputed commission. I feel constrained, however, to add a few brief observations.
The principal issue confronting this Court is whether the court below erred in denying appellant’s motion for judgment on the pleadings while at the same time granting appellee’s similar motion. We are compelled, therefore, to concern ourselves exclusively with the pleadings,1 and, more specifically, with whether appellee, while admitting for the purposes of his motion the truth of all appellant’s allegations and the untruth of his own allegations which have been properly denied,2 has established a right to enforcement of his claim.
While agreeing in principle, the dissent urges that this Court reverse the judgment of the court below on the ground that appellee, irrespective of appellant’s answer, has failed to plead the “material facts” requisite to the averment of an enforceable contract. Since it is not contended by the majority that appellee’s right to recovery may be predicated on a third party beneficiary theory,3 it is necessary under the view there taken that the appellee have averred that he was a *287party to tbe written contract upon which his claim is predicated. And this, it is urged by the dissent, he has failed to do in that it is claimed that his sole manifestation of assent to the writing is his signature in his representative capacity. Concluding that appellee has not otherwise averred the manner by which his assent — in his individual capacity — was manifested, it is asserted that appellee has failed to aver that he was a party to the contract and properly to plead his claim.
The record supports no such conclusion. I believe, as does the majority, that appellee has adequately averred the manner of his assent, and, therefore, that he is a party to the contract upon which he claims.
The contract which appellee has averred is a unilateral contract. That much appears evident from the pleadings.4 In such a contract, the very act of per*288forming that which has been requested by the offeror is both the consideration for the promise sought to be enforced and the acceptance of the offer for the contract. Restatement, Contracts, §§52, comment a, 56, comment a (1932); 1 Williston, Contracts, §65 (3d ed. 1957); see Quilty v. New York Life Ins. Co., 153 Kan. 129, 109 P. 2d 215 (1941); Gunnison v. Evans, 136 Kan. 791, 18 P. 2d 191 (1933); Lineaweaver's Estate, 284 Pa. 384, 131 Atl. 378 (1925); 1 Corbin, Contracts §56 (1963). Moreover, as the objective manifestation of the acceptance of the offer for the contract, performance of the requested act is at the same time a manifestation of assent to it. See ibid.
It follows, then, in the instant case, that if appellee has adequately averred the performance of the requested act, the securing of a tenant under terms satisfactory to appellant, then he has properly averred that he is a party to the written contract upon which he claims. Cf. Restatement, Contracts, §12 (1932).
No searching examination of appellee’s pleadings is necessary to discern the required averment of performance. In his complaint, appellee alleges the execution of a lease which was ratified and approved by appellant, a matter which is not disputed. The conclusion which one is necessarily drawn to is that appellee has, through this allegation, averred the performance of the act requested by appellant and, by necessary implication, his assent to the written contract.
*289It remains to be determined whether, in light of appellant’s answer, appellee was entitled to judgment on the pleadings.
Appellant contends, inter alia, (1) that the sales commission was not part of any understanding or agreement between the parties, and (2) that the provision with respect to the disputed commission was not deleted from the executed agreement by reason of mistake.5
As the majority has correctly stated, the allegation of mistake fails to conform to the rule that such allegations be pleaded with particularity. Pa. R. C. P. 1019(b); Lefkowitz v. Hummel Furniture Co., 385 Pa. 244, 248, 122 A. 2d 802, 804 (1956); 1 Goodrich-Amram §1019 (b)-1 (1960); 4 Standard Pennsylvania Practice §74 (1955). Lacking such particularity, appellant’s unsupported allegation is a mere legal conclusion and as such may be disregarded. Lefkowitz v. Hummel Furniture Co., supra; see Architectural Tile Co. v. McSorley, 311 Pa. 299, 166 Atl. 913 (1933); 4 Standard Pennsylvania Practice §125 (1955). It only remains to determine whether appellant’s denial that the sales commission was a part of the understanding of the parties is a barrier to the relief granted appellee by the court below.
Appellant neither disputes the authenticity of the lease nor its execution. The lease agreement contains in clear and uncertain terms the provision upon which appellee seeks recovery. In light of these facts, the crux of appellant’s averment can only be reduced to the contention that the agreement is legally unenforceable or that its inclusion was by reason of mistake. Neither averment, however, is sufficient to bar judgment on the pleadings. The latter allegation is insufficient for the reasons stated above. The former aver*290ment is no less a conclusion of law and obviously has no operative force once appellee has pleaded facts requisite to the enforcement of a contract. See, e.g., Smith v. Brown-Borhek, 414 Pa. 325, 330, 200 A. 2d 398, 400 (1964); Bednarowicz v. Vetrone, 400 Pa. 385, 387-88, 162 A. 2d 687, 688 (1960); Robinson v. Philadelphia, 400 Pa. 80, 82, 161 A. 2d 1, 2 (1960); London v. Kingsley, 368 Pa. 109, 111, 81 A. 2d 870, 871 (1951); Architectural Tile Co. v. McSorley, 311 Pa. 299, 166 Atl. 913 (1933); 4 Standard Pennsylvania Practice §125 (1955).
Appellant, no less than appellee, is under an obligation to plead material facts, which if established, would constitute a barrier to the enforcement of appellee’s claim. Not having done so, appellant is not entitled to the trial of phantom issues. Having had the opportunity to raise an issue for trial and having failed to do so, appellant may not now complain of the granting of appellee’s motion for judgment on the pleadings.
Nederostek v. Endicott-Johnson, 415 Pa. 136, 138, 202 A. 2d 72, 73 (1964); Emery v. Metzner, 191 Pa. Superior Ct. 440, 445. 156 A. 2d 627, 630 (1959); Bogojavlensky v. Logan, 181 Pa. Superior Ct. 312, 320, 124 A. 2d 412, 416-17 (1956); 2 A Anderson. Penna. Civil Practice §1034.21 (Supp. 1964); 1 Goodrich-Amram §§1034(a)-1, 1034(a)-3, 1034(b)-1 (1960).
Nederostek v. Endicott-Johnson, 415 Pa. 136, 138, 202 A. 2d 72, 73 (1964); Smith v. Brown-Borhek Co., 414 Pa. 325, 331, 200 A, 2d 398, 400 (1964); Necho Coal Co. v. Denise Coal Co., 387 Pa. 567, 568, 128 A. 2d 771, 772 (1957); Cary v. Lower Merion School Dist., 362 Pa. 310, 312, 66 A. 2d 762, 763 (1949); 1 Goodrich-Amram §1034(b)-1 (1960).
On this point I venture no opinion as I see no need to reach the issue.
In his complaint, appellee averred that while acting in a representative capacity, as agent for appellant, he entered into a lease agreement with one Sailor, which lease agreement was ratified and approved in writing by appellant. He further alleged that the lease agreement contained, in a provision numbered paragraph 37(B), certain agreements between appellant and appellee in his individual capacity. Paragraph 37(B) of the written lease agreement, which appellee annexed to his complaint and which, thus, became part of his pleadings, see 1 Goodrieh-Amram §1019 (g)-2 (1960), provides: “In consideration of the services of . . . [appellee) in securing the execution of the above lease . . . [appellant] authorizes . . . [appellee] to collect the rent due . . . thereunder . . . and to deduct and retain five per cent (5%) of the amount collected each month .... In the event that ... the premises shall be sold to the Lessee . . . [appellant] agrees to pay to . . . [appellee] a commission of 5% of the sale price . . . .”
In my view, the practical effect of appellee’s incorporation of the lease agreement, and more particularly paragraph 37(B), is the averment that in consideration for appellant’s promise to pay the therein provided for commissions, appellee secured the execution of a lease agreement under terms satisfactory to appellant. Whether the contract should be more properly characterized as a so-called reverse unilateral contract, see Restatement, Contracts, §57 (1932); 1 Williston, Contracts §71 (3d ed. 1957), is of little *288moment. I find it conceptually more profitable to view any overtures made by the promisee of a unilateral contract as preliminary negotiations rather than an offer in that I am hard pressed to accept the notion that what is not enforceable as a promise may be fairly characterized as an “offer.” But even viewed as a reverse unilateral contract, the result is unaffected. The incorporated lease may then be fairly viewed as an averment that appellee offered to secure the execution of a satisfactory lease agreement in consideration for certain commissions and that appellee accepted that offer by reason of his ratification of the lease agreement.
Appellant raises a number of other contentions which were summarily and properly disposed of by the majority.