not taken so as to invoke the protection of article I, section 17 of the Texas Constitution. The Dillards, in four points of error, argue that governmental immunity does not bar claims based on: (1) promissory and equitable estoppel; (2) a taking under article I, section 17 of the Texas Constitution; (3) fraud; and (4) breach of the duty of good faith negotiation.
When a defendant moves for summary judgment on the basis of an affirmative defense such as governmental immunity, it must conclusively prove all essential elements of that defense. Swilley v. Hughes, 488 S.W.2d 64, 67 (Tex.1972). AISD, thus, had the burden of showing that no genuine issue of material fact existed and that it was entitled to judgment as a matter of law. Nixon v. Mr. Property Management Co., 690 S.W.2d 546, 548 (Tex.1985).
1. GOVERNMENTAL IMMUNITY
In their first point of error, the Dillards claim the trial court erred in holding that governmental immunity barred the application of the doctrines of promissory and equitable estoppel against AISD. We agree with the trial court.
A. General Principles
Governmental immunity consists of two basic principles of law.2 First, the state as sovereign is immune from suit without consent even though there is no dispute regarding the state’s liability. Missouri Pac. R.R. v. Brownsville Navigation Dist., 453 S.W.2d 812, 813 (Tex.1970); see, e.g., Hosner v. De Young, 1 Tex. 764, 769 (1847); Board of Land Comm’rs v. Wall ing, Dallam 524, 525-26 (Tex.1843). The doctrine bars a suit against the state unless the state has expressly given its consent to be sued. See, e.g., Missouri Pac. R.R. at 814 (statute creating a navigation district that states that the district can “sue and be sued in all courts of this state” is sufficient consent to sue the state); Texas Tort Claims Act, Tex.Civ.Prac. & Rem.Code Ann. § 101.025 (1986) (person having a claim under the Tort Claims Act is granted permission to sue) [hereinafter Tort Claims Act]; Tex.Civ.Prac. & Rem.Code Ann. §§ 107.001-.005 (Supp.1991) (legislative resolution granting permission to sue the state).
Second, the state has immunity from liability even though the state has consented to be sued. Missouri Pac. R.R., 453 S.W.2d at 813; State v. Isbell, 127 Tex. 399, 94 S.W.2d 423, 424 (1936); Tex.Civ.Prac. & Rem.Code Ann. § 107.002(b) (Supp.1991) (legislative resolution granting permission to sue the state does not waive immunity from liability); see, e.g., Tort Claims Act §§ 101.021, .023, .025(a), .107 (1986 & Supp. 1991). An important corollary is that the state is generally not liable for the acts of public servants. E.g., Lowe v. Texas Tech Univ., 540 S.W.2d 297, 298 (Tex.1976); Whitfield v. City of Paris, 84 Tex. 431, 19 S.W. 566, 567 (1892) (doctrine of respondeat superior does not apply against the state); City of Galveston v. Posnainsky, 62 Tex. 118, 125 (1884); Tex.Civ.Prac. & Rem.Code Ann. §§ 104.001-008 (1986 & Supp.1991) (state liability for conduct of public servants).
A critical exception, however, is that the state waives its immunity from liability when it contracts:
It is well settled that so long as the state is engaged in making or enforcing laws, or in the discharge of any other governmental function, it is to be regarded as a sovereign, and has prerogatives which do not appertain to the individual citizen; but when it becomes ... a party to a contract with a citizen, the same law applies to it as under like conditions governs the contracts of an individual.
Fristoe v. Blum, 92 Tex. 76, 45 S.W. 998, 999 (1898); see State v. Elliott, 212 S.W. 695, 698 (Tex.Civ.App.1919, writ ref’d). The state is still immune from suit sounding in contract, however, even though the state has consented to liability by the act of contracting. Elliott, 212 S.W. at 698 ("Because of her sovereignty, the state is not
2
The supreme court has held that the defense of governmental immunity is waived if not affirmatively pleaded. Davis v. City of San Antonio, 752 S.W.2d 518, 520 (Tex.1988).