This writ petition is mainly directed against the assessment order for the periods ending 31.3.89 and 30.9.89 under the Assam Finance (Sales Tax) Act, 1956. 2. The petitioner is a registered dealer under the Assam Finance (Sales Tax) Act, 1956 and submitted its returns of turnover before the Superintendent of Taxes. According to the petitioner/company, it is claimed for exemption from payment of sales tax on the basis of Eligibility Certificate issued by the Udyog Sahayak and accordingly, no tax was deposited. The assessing authority rejected the claim of the petitioner since the unit is an old unit and the assessment was accordingly made. Two revision petitions were presented before the Joint Commissioner of Taxes questioning the order of assessment on the .face of the Eligibility Certificate granted by the Udyog Sahayak. The Joint Commissioner of Taxes, upon hearing the parties and on considering the materials on record, held that only the new industrial units were entitled of exemption from payment of taxes as per provisions of the Assam Industries (Sales Tax Concession) Act, 1986. Since the exemptions from payment of sales tax were permissible only to the new units in which effective steps for establishment of the industries were taken on or after 15.10.82 but before 31.3.90, which was not the case so far as it related to the petitioner/company, the Joint Commissioner dismissed the revision petitions. Hence this writ petition assailing the legality of the order dated 4.11.1992. 3. The petitioners mainly based their claim of exemption on the basis of the Incentive Scheme of 1986 announced by the Govt of Assam which provided amongst others, for granting of sales tax exemption to the specified units. As per the scheme, the new package of incentives was operative for the period from 1.1.87 to 31.3.90 or till such time that the Govt may deem it fit to continue. As per the Scheme, only the new units set-up on or after 1.1.87 and the existing units undertaking expansion, modernisation or diversification at the same location or at any other place in the State of Assam, would be eligible for incentives under the 1986 Incentive Scheme on fulfillment of the conditions specified therein. As per the Scheme, only the new units set-up on or after 1.1.87 and the existing units undertaking expansion, modernisation or diversification at the same location or at any other place in the State of Assam, would be eligible for incentives under the 1986 Incentive Scheme on fulfillment of the conditions specified therein. The Scheme provides that expansion/modernisation/diversification of a existing industrial unit will also be eligible for the incentive if the total capital investment on plant and machinery for expansion/modernisation or diversification, as the case may be, was more than twenty five percent of the total fixed capital investment of the existing unit. The Scheme itself indicated that no right or claim for any incentive under the Scheme was conferred merely on fulfillment of the conditions specified in the Scheme. The Scheme indicated about the Eligibility Certificates and the implementing agencies. One of the incentives under the aforesaid Scheme of 1986, related of sales tax exemption. Sales tax was exempted on purchase of raw materials and sales of finished products for a period of five years as may be decided by the Govt from time to time. According to the petitioners, said company fulfilled all the requirements and in fact the company was favoured with an Eligibility Certificate which was valid from 1.6.88 to 1.6.89. The Udyog Sahayak issued another Eligibility Certificate to the petitioner/company in continuation of the earlier Eligibility Certificate granting exemption from payment of sales tax for a period of five years from 1.6.88 to 28.2.93. 4. In the writ petition, it is contended that as per the Scheme announced by the Govt of Assam, the petitioner/company is entitled to exemption from payment to sales tax. The said Scheme constitutes a definite promise and assurance that the industrial units like that of the petitioners, will be entitled to sales tax exemption. On the basis of the assurance of the respondents, the petitioner/company substantially altered its position and made huge investments in the business. The nature and the degree and range of the substantial alteration of the position is, however, not indicated in the writ petition. The gravamen of the plea is based on the plea of promissory estoppel. On the basis of the assurance of the respondents, the petitioner/company substantially altered its position and made huge investments in the business. The nature and the degree and range of the substantial alteration of the position is, however, not indicated in the writ petition. The gravamen of the plea is based on the plea of promissory estoppel. The petitioners concedes that under the Sales Tax laws including the Assam Industries (Sales Tax Concession) Act, 1986, the petitioners are not entitled for any sales tax concessions, they only on their hope for exemption on the strength of the 1986 Industrial Policy. Long time after the institution of the civil rule, the petitioners filed an application for amendment of the petition whereby they sought to challenge the Assam Industries (Sales Tax Concessions) Scheme, 1995. Since the main relief sought by the petitioners is relating to the assessment of taxes prior to coming into force of the 1995 Scheme, the question of going into those aspects of the matter in this writ petition does not arise. At any rate, the validity of the Assam Industries (Sales Tax Concessions) Scheme, 1995 came up for consideration in a number of civil rules and this Court by its judgment and order dated 29.4.98, passed in Manjushree Extrusion Ltd & others vs. State of Assam & others, reported in 1998 (2) GLT164 (1998 (2) GLJ 24), upheld the scheme. Simply because the 1995 Scheme which was made effective from 1.4.91 granting benefits to new industries established on or after 1.4.91 and the existing industries which existed prior to 1.4.91 undertaking expansion/modernisation and diversification work and in fact that the 1995 Scheme took care for the incentives as envisaged in the 1995 Industrial Policy of the Govt of Assam, the same benefit cannot be extended on the asking of the petitioners to the industries undertaking expansion/modernisation and diversification work as per the 1986 Industrial Policy and the Incentive Scheme of the Govt of Assam. The plea of discrimination on this ground in untenable. 5. Estoppel is a rule of evidence binding one private party against the other. Section 115 of the Evidence Act accepts the principle of estoppel. The plea of discrimination on this ground in untenable. 5. Estoppel is a rule of evidence binding one private party against the other. Section 115 of the Evidence Act accepts the principle of estoppel. The above provision envisage that: "When one person, by his declaration, act or omission, intentionally caused or permitted another person to believe a thing to be true and to act upon such belief, neither he nor his representative shall be allowed, in any suit or proceeding between himself and such person or his representatives, to deny the truth of that thing." Estoppel conveys the idea that a person who by his declaration, act or omission induces one person to act to this detriment, in reliance on the truth of it, is not permitted to deny it later. It is not only a rate of evidence, but it has become a principle of law. The rule of estoppel is similarly applicable to the public authorities. Justice, fairness and equity come to the rescue against a breach of the promise. It may however be kept in mind that the application of the principle of promissory estoppel will entirely depend on the setting of the fact. The plea of Estoppel cannot overlook the free and legitimate exercise of the power by authorities or due performance of the statutory duties in the public interest. The plea of estoppel cannot stand in the way of legitimate exercise of statutory powers, nor the estoppel can bind any promise or undertaking which is ultra vires. Similarly the principle of estoppel cannot come in the way of the Govt in formulating its policy and the Legislature from making statutory provisions. Simply because many people have become victims of vicissitude of Govt policy, the same cannot hinder the State from formulating and reformulating its policy in the public interest. Sir William Wade in the Seventh Edition of the 'Administrative Law', referred to the statement of Lord Lawton in the case of Laker Airways Ltd. vs. Department of Trade [(1977) QB 643] which reads as follows : "A Government department which encourages an airline to invest in aircraft on the understanding that its licence will be continued is not estopped, if there is a change of Government and a reversal of policy, from withdrawing the licence. Many people may be victims of political vicissitudes and 'estoppel cannot be allowed to hinder the formation of Government policy'." Late Prof Stanely De Smith in his 'Judicial Review of Administrative Action' culled down the following principles in respect of Public Law powers : "(1) A public body with limited powers cannot bind itself to act outside of its authorised powers; and if it purports to do so it can repudiate its undertaking, for it cannot extend its powers by creating an estoppel. A body entrusted with duties or with discretionary powers for the public benefit effectively may not avoid its duties or fetter itself in the discharge of its powers (including duties to exercise its powers free from extraneous impediments). The general rules of agency apply in public law, except that an agent (a) cannot bind his principal to do what is ultra vires and probably (b) cannot bind his principal by exceeding his own authority if that authority is circumscribed by statute. Purported authorisation, waiver, acquienscence and delay do not preclude a public body from reasserting its legal rights or powers against another party if it has no power to sanction the conduct in question or to endow that party with the legal right or immunity that he claims. However, there are dicta to the effect that planning authorities may waive certain defects in formal procedural requirements, perhaps so long as third parties are not adversely affected. In respect of unauthorised and erroeous assurances or advice given by officials upon which members of the public rely to their detriment, there have been two approaches. At one time it could be safely said that there were simply nugatory (unless they fell within the scope of agency in contract), although a negligent misstatement or course of conduct causing economic loss might give rise to liability in tort. At one time it could be safely said that there were simply nugatory (unless they fell within the scope of agency in contract), although a negligent misstatement or course of conduct causing economic loss might give rise to liability in tort. Thus if a local Govt officer to whom the necessary powers have not been delegated assures a builder that planning permission is not required for what he proposes to do, this assertion, though acted upon by the inquirer, does not affect the power of the local authority to arrive at and act on an opposite decision Where a person who has relied to his detriment upon erroneous advice has suffered loss, and where the doctrine of estoppel will not come to his aid, administrative redress will in any event often be a more appropriate remedy than judicial review, and complaints can be investigated by the appropriate Ombudsman as allegations of injustice caused by maladministration. " Principle of promissory estoppel cannot estop public authorities from discharging its lawful duties and powers conferred on them by statutes and estoppel cannot be pleaded against the promised activities of a public authority which is per se unlawful or unauthorised. Similarly, where a person who has relied to his detriment on erroneous advice and suffers loss, the doctrine of estoppel will come to his aid. The doctrine of promissory estoppel is based on equity and to avoid injustice. The doctrine cannot affect operation of law. In the case of Motilal Padampat Sugar Mills vs. State of UP, reported in (1979) 2 SCC 409 , it was observed that the doctrine of promissory estoppel would not apply in the teeth of obligation or liability imposed by law and there can be no promissory estoppel against the exercise of legislative powers. The facts of this case are not analogous to those as found in the cases- Union of India vs. Anglo Afgan Agencies Ltd. (reported in AIR 1968 SC 718 ), Motilal Padamapat Sugar Mills vs. State of UP (supra), Purnami Oil Mills vs. State of Kerala (reported in (1986) (Supp) SCC 728), Union of India vs. Godfrey Philips India (reported in (1985) 4 SCC 369 ); and therefore, the principle of law as laid down therein, is not applicable to this case. 6. 6. I have already indicated the nature of the pleadings relied on by the petitioners which contain only the bald expressions without being supported by materials facts. The doctrine of estoppel cannot be applied in the air and the Court is duty bound to consider all the aspects of the matter including the public good at large. In considering the applicability of the doctrine, the Court is concerned with the fundamental principles of equity. As pointed out earlier, the petitioners have basically focussed against the order of assessment and collaterally attacked the industrial policy. On perusal of the materials on record also, I do not find any infirmity in the order of assessment including the order dated 4.12.92, passed by the Joint Commissioner of Taxes, Assam in the revision petitions requiring interference from this Court. In view of above, the writ petition deserves to be dismissed and thus the same dismissed with a cost of Rs.5,000/- (Rupees five thousand) only. The interim order dated 28.7.93 stands vacated.