Public purpose is the heart of a tax law

The purpose of a law is evident from its text or inferable from other secondary sources. 

The term public purpose is not defined. It is an elastic concept that can be hammered to fit modern standards. Jurisprudence states that public purpose should be given a broad interpretation. It does not only pertain to those purposes which are traditionally viewed as essentially government functions, such as building roads and delivery of basic services, but also includes those purposes designed to promote social justice. Thus, public money may now be used for the relocation of illegal settlers, low-cost housing and urban or agrarian reform.

While the categories of what may constitute a public purpose are continually expanding in light of the expansion of government functions, the inherent requirement that taxes can only be exacted for a public purpose still stands. Public purpose is the heart of a tax law. When a tax law is only a mask to exact funds from the public when its true intent is to give undue benefit and advantage to a private enterprise, that law will not satisfy the requirement of public purpose. (G.R. No. 166006)