227 F.3d 928 (7th Cir. 2000)
In re Vic Supply Company, Inc., Debtor.Falconbridge U.S., Inc., Plaintiff-Appellant,v.Bank One Illinois, N.A., Defendant-Appellee.
No. 99-4110
In the  United States Court of Appeals  For the Seventh Circuit
Argued May 18, 2000Decided September 19, 2000

Appeal from the United States District Court  for the Northern District of Illinois, Eastern Division.  No. 99 C 2714--Elaine E. Bucklo, Judge.
Before Posner, Diane P. Wood, and Williams, Circuit  Judges.
Posner, Circuit Judge.


1
This case involves a  dispute between two creditors of the bankrupt Vic  Supply Company. Bank One had a security agreement  with Vic, covering all of Vic's assets.  Falconbridge sold Vic nickel for resale,  acquiring a purchase money security interest  covering the proceeds of the resale. The  bankruptcy court, seconded by the district court,  to which Falconbridge had appealed, ruled that  Bank One's security interest was prior to  Falconbridge's under Article 9 of the Uniform  Commercial Code (codified in Illinois as 810 ILCS  5/9). Falconbridge argues that its interest is  prior because the security agreement between the  bank and Vic was never accepted by the bank. The  argument turns on whether a secured lender's  failure to sign such an agreement, when the  agreement provides that it is effective only when  signed by the lender, allows a subsequent secured  lender to take priority over the earlier one.


2
The bank began lending money to Vic in 1980, and  that year it filed a UCC financing statement with  the Secretary of State of Illinois covering all  of Vic's inventory, accounts receivable, and  equipment. The statement implied that the parties  had or intended soon to make an agreement  granting the bank a security interest in these  assets, the purpose of filing such a statement  being to perfect such an interest. The UCC does  not require, for a security interest to be  perfected, that the security agreement precede  the filing of the financing statement, sec. 9-  402, let alone that it be attached to the  statement (the statement "indicates merely that  the secured party who has filed may have a  security interest in the collateral described,"  sec. 9-402, official comment 2; see In re Varney  Wood Products, Inc., 458 F.2d 435 (4th Cir.  1972)), although that is often done; and we do  not know the agreement's terms.


3
Ten years later, with the bank continuing to  extend credit to Vic, Vic signed another  agreement, granting the bank a continuing  security interest in all Vic's assets. The  agreement expressly covered--what was anyway  implicit, sec. 9-306(2)--the proceeds of sales  from inventory. A few months earlier the bank had  filed with the Illinois Secretary of State a  continuation financing statement materially  identical to the one filed back in 1980.


4
Years passed, and Falconbridge, having obtained  a purchase money security interest in the nickel  that it was selling to Vic, filed a financing  statement, just as Bank One had done. This  security interest, like Bank One's, automatically  extended to the proceeds of Vic's resale of the  nickel. When Vic later defaulted, Bank One's  security interest, having been recorded before  Falconbridge's, would normally have had priority,  sec. 9-312(5)(a); Falconbridge was not entitled  to the superpriority that is accorded to a  purchase money security interest when the debtor  receives cash proceeds in a sale of the  collateral, sec. 9-312(3), because the proceeds  that Vic received were in the form of accounts  receivable rather than cash.


5
But Bank One had failed to sign the 1990  security agreement, and Falconbridge argues that  this failure means that the agreement did not  become effective. The agreement provides that  "the terms and provisions of this agreement shall  not become effective and Bank shall have no  duties hereunder unless and until this agreement  is accepted by Bank as provided below"--and below  is a blank for a signature that was never filled  in. Falconbridge reminds us that "a security  agreement is effective according to its terms  between the parties, against purchasers of the  collateral and against creditors." UCC sec. 9-  201. "According to its terms," Falconbridge  argues, the agreement never came into effect.  Although the agreement authorizes the bank to  fill in any blanks in it, the bank had not done  that; and according to Falconbridge, it was too  late for the bank to do so once Falconbridge  perfected its own security interest by filing a  UCC financing statement. In short, Falconbridge  argues, the bank had no security interest when  Falconbridge acquired its own security interest.


6
Ordinarily, only a party (actual or alleged) to  a contract can challenge its validity, e.g.,  Williams v. Eggleston, 170 U.S. 304, 309 (1898);  IDS Life Insurance Co. v. SunAmerica, Inc., 103  F.3d 524, 529 (7th Cir. 1996); OPE Shipping Ltd.  v. Allstate Ins. Co., 687 F.2d 639, 643 (2d Cir.  1982); Tri-Star Pictures, Inc. v. Leisure Time  Productions, B.V., 749 F. Supp. 1243, 1250  (S.D.N.Y. 1990), and neither party to the  security agreement that is at issue in this case-  -neither Bank One nor Vic--challenges the  validity of the agreement. Of course there are  illegal contracts that the government, or persons  injured by the contract, can challenge, such as a  contract in restraint of trade, but there is no  suggestion of that here. Obviously the fact that  a third party would be better off if a contract  were unenforceable does not give him standing to  sue to void the contract. A contract that while  not unenforceable by either party, or within the  class of contracts deemed illegal, might,  intentionally or negligently, deceive a third  party, who in that event might have a tort  remedy. But there is no suggestion that the  defect in the bank's security agreement with Vic-  -the absence of the bank's signature--harmed  Falconbridge. So far as appears, Falconbridge was  unaware of the defect or of anything else about  the agreement. It knew, or at least thought,  there was an agreement, because before extending  credit to Vic it checked the UCC registry and  read the bank's financing statement, which  notified Falconbridge that the bank had a  security interest, implying, as we said, that it  had a security agreement with Vic. Falconbridge  even wrote Bank One that it was planning to  obtain a purchase money security interest in the  nickel it was selling Vic and in any proceeds of  resales of the nickel. The bank cannot be accused  of having misled Falconbridge. National Bank v.  Haupricht Bros., Inc., 564 N.E.2d 101, 114 (Ohio  App. 1988) (per curiam); cf. Elhard v. Prairie  Distributors, Inc., 366 N.W.2d 465, 468 (N.D.  1985).


7
So Falconbridge cannot appeal to any general  legal or equitable principle that might enable it  to challenge the validity of the bank's agreement  with Vic. But we must consider the bearing of UCC  sec. 9-203(1)(a), which provides that a security  interest is not "enforceable against the debtor  or third parties with respect to the collateral"  unless (the collateral not being in the  creditor's possession or control) "the debtor has  signed a security agreement which contains a  description of the collateral." Some courts have  permitted a subsequent creditor to use this  provision to knock out the priority of an earlier  creditor without requiring proof that he was  actually misled or otherwise prejudiced by any  defect in the security agreement. World Wide  Tracers, Inc. v. Metropolitan Protection, Inc.,  384 N.W.2d 442 (Minn. 1986); In re Middle  Atlantic Stud Welding Co., 503 F.2d 1133, 1136  (3d Cir. 1974). This result, which is contrary to  the Haupricht case cited earlier, strikes us as  questionable despite its conformity to the  literal language of section 9-203; it gives the  later creditor a windfall by enabling him to  knock out a priority on the basis of a defect on  which he did not rely. No matter. The debtor  signed the security agreement in this case and  the agreement describes the collateral. The fact  that section 9-203 requires the debtor to sign  and does not mention signature by the creditor  helps to show that the draftsmen did not think  that a priority should be lost merely because the  creditor's signature was missing.


8
A complicating factor in some cases, though not  in this one, is that a trustee in bankruptcy (or  debtor in possession) has the status of a  hypothetical lien creditor "without regard to any  knowledge of the trustee or of any creditor," 11  U.S.C. sec. 544(a), entitling him to void a  security interest because of defects that need  not have misled, or even have been capable of  misleading, anyone. Pearson v. Salina Coffee  House, Inc., 831 F.2d 1531 (10th Cir. 1987); In  re Sandy Ridge Oil Co., 807 F.2d 1332 (7th Cir.  1986); Sommers v. International Business  Machines, 640 F.2d 686, 688-89 (5th Cir. 1981).  The trustee did not assert this right here,  probably because the only benefit would have been  to another secured creditor, Falconbridge, and  the trustee's duty is to maximize the recovery of  the unsecured creditors.


9
For completeness we further note that while the  provision requiring an adequate description of  the collateral is intended in part for the  protection of subsequent creditors, In re Martin  Grinding & Machine Works, Inc., 793 F.2d 592,  596-97 (7th Cir. 1986); In re Laminated Veneers  Co., 471 F.2d 1124, 1125 (2d Cir. 1973), and so  may be enforced by them, at least when they can  show they were misled by an inadequate  description, the provision requiring the debtor's  signature is not intended for their protection.  It is a statute of frauds provision intended to  make it easier for courts to resolve disputes  over the meaning of the agreement. 2 James J.  White & Robert S. Summers, Uniform Commercial  Code sec. 24-4 at 305 (1988). Falconbridge had no  standing to enforce it. See UCC sec. 2-201,  official comment 4; cf. BDS, Inc. v. Gillis, 477  A.2d 1121, 1123 (D.C. App. 1984) (per curiam); 2  E. Allan Farnsworth, Farnsworth on Contracts sec.  6.10, p. 168 (1990).


10
The real significance of section 9-203 in this  case is in helping to explain section 9-201. In  providing that a security agreement is effective  only according to its terms, the draftsmen meant  to state the general rule to which section 9-203  creates an exception. See UCC sec. 9-201,  official comment. An agreement that violates  section 9-203 may not be effective according to  its terms. But that is not the only function of  section 9-201. For the fact that Falconbridge is  not permitted to question that the bank had a  security interest does not automatically  extinguish Falconbridge's security interest. That  depends on the relative priority of the  contestants' security interests, and this is  where section 9-201 could bite in this case: the  bank can enforce a security agreement against a  subsequent creditor of its debtor, that is, can  retain its priority, only insofar as the  agreement gives it a security interest. Had the  agreement granted the bank a security interest  only in inventory by expressly disclaiming any  grant of a security interest in proceeds as well,  the bank could not claim the proceeds in  derogation of Falconbridge's later-perfected  security interest in them. But there is nothing  like that here. There is no discrepancy between  the bank's financing statement and its security  interest. Both cover all the assets of Vic,  leaving nothing for Falconbridge until the bank's  claim has been satisfied.


11
Although unnecessary to add, the security  agreement was valid; that is, Vic would have had  no defense against enforcement of the agreement  by the bank, or vice versa. For one thing, it is  apparent from the wording of the signature  requirement and the fact that the bank was  authorized to fill in any blank in the agreement  that the requirement was intended solely for the  bank's protection, and was not intended to confer  any right on Vic; it was a defect of which no one  could complain. For another thing (but it is  actually closely related), after the agreement  was not signed the bank lent money to Vic against  its inventory nonetheless, and the parties  assumed that this credit was pursuant to the  terms of the security agreement. Acceptance can  be effectuated by performance as well as by a  signature. Restatement (Second) of Contracts sec.  30(2) (1981); 1 Farnsworth, supra, sec. 3.12, p.  222; see also UCC sec. 2-206(1)(a); 1 White &  Summers, supra, sec. 1-5, p. 55. And while  parties can specify that performance shall not be  effective as acceptance, Golden Dipt Co. v.  Systems Engineering & Mfg. Co., 465 F.2d 215 (7th  Cir. 1972); In re Newport Plaza Associates, L.P.,  985 F.2d 640, 645 (1st Cir. 1993); Restatement,  supra, sec. 30, comment a, this would be an  implausible interpretation of the acceptance  clause that we quoted earlier. It would amount to  saying that if the parties had been asked, "if  the bank fails to sign the agreement, will the  agreement be void even if the parties behave in a  way that shows they thought it was in effect?"  they would have said "yes." Or that if they had  been asked, "does the bank's failure to sign mean  that the debtor could repudiate the agreement at  any time?" they would have said "no." What they  really would have said would have been, "don't be  silly; it was just an oversight, of no  significance--and anyway the requirement of the  bank's signature was for the protection of the  bank, not of the debtor."


12
Falconbridge's argument that the bank's effort  to show acceptance by performance violates the  parol evidence rule (which both parties call the  "parole evidence" rule) is frivolous. The rule  bars evidence oral or written concerning  negotiations leading up to an integrated  contract, not evidence of events subsequent to  the writing that is claimed to be the statement  of the parties' contract. Fischer v. First  Chicago Capital Markets, 195 F.3d 279, 282 (7th  Cir. 1999); Williams v. Jader Fuel Co., 944 F.2d  1388, 1394 (7th Cir. 1991); 2 Farnsworth, supra,  sec. 7.6 p. 228. Anyway, to repeat a theme of  this opinion, the parol evidence rule, like other  contract defenses, is intended for the protection  of parties or alleged parties to contracts; it is  not intended to enable a stranger to break up a  contractual relation. Northern Assurance Co. v.  Chicago Mutual Building Ass'n, 64 N.E. 979 (Ill.  1902); Quality Lighting, Inc. v. Benjamin, 592  N.E.2d 377, 382 (Ill. App. 1992); Rittenhouse v.  Tabor Grain Co., 561 N.E.2d 264, 271 (Ill. App.  1990). So there was acceptance; but if there  hadn't been, yet the parties agreed they had a  contract, that would be enough to defeat  Falconbridge's effort to invalidate a contract on  which (or on the defect in which) it did not rely  in extending credit to Vic.


13
Affirmed.


14
Williams, Circuit Judge, concurring.


15
I agree with  my colleagues that Falconbridge cannot challenge  the validity of the security agreement between  Bank One and Vic Supply. I write separately to  voice my concern regarding the majority's further  determination that the security agreement is, in  any event, valid. While I am sympathetic to the  majority's intuition that Bank One and Vic Supply  could not have intended the signature provision  to mean what it says, I am not persuaded that  such an intuition justifies reading the provision  out of the agreement.


16
Under the signature provision, the security  agreement becomes effective only if and when Bank  One accepts the agreement by filling in the  signature blank; something Bank One did not do.  The signature provision is not at all ambiguous,  nor is there any reason to believe it is merely  surplusage. It is a cardinal principal of  contract law that since the language of a  contract is the best evidence of the parties'  intent, every provision of a contract should be  given content and effect, and unambiguous  contractual language should be given its plain  and natural meaning. See, e.g., Emergency Med.  Care, Inc. v. Merion Mem'l Hosp., 94 F.3d 1059,  1061 (7th Cir. 1996) (applying Illinois law);  River Forest State Bank & Trust Co. v. Rosemary  Joyce Enters. Inc., 689 N.E.2d 163, 167 (Ill.  App. Ct. 1998). It does not matter that in  hindsight one or both parties (or a court) might  have second thoughts about the wisdom of  including a particular term. Our task is to  enforce the terms the parties included in their  contract.


17
To my mind, moreover, it is telling that the  majority cites not a single case adopting the  same pragmatic approach it employs in  interpreting the signature provision of the  security agreement. No rule of law of which I am  aware allows us to disregard the unambiguous  terms of a contract in favor of what we believe  the parties must have intended. Again, our task  is to enforce the terms the parties included in  their contract. Accordingly, I cannot join in the  majority's conclusion that the security agreement  between Bank One and Vic Supply is valid

