Courts have progressively read Directive Principles of State Policy into Article 21, converting non-justiciable welfare obligations into enforceable fundamental rights. Through PIL, judicial review, and harmonious construction, the Supreme Court expanded the right to life to include livelihood, education, health, environment, and legal aid. This shift moved the relationship between Part III and Part IV from conflict — where Fundamental Rights prevailed — to complementarity, treating both as equally indispensable constitutional pillars.

The Constitution creates a deliberate asymmetry between Part III and Part IV. Fundamental Rights are justiciable — courts can strike down laws that violate them under Articles 32 and 226. Directive Principles are explicitly non-justiciable under Article 37: the State must apply them in governance, but no court can enforce them directly.

Early judicial interpretation treated the two as potentially conflicting, with Fundamental Rights prevailing in case of clash (State of Madras v. Champakam Dorairajan, 1951). Parliament responded with constitutional amendments; courts countered with the basic structure doctrine from Kesavananda Bharati (1973), which held that even amendments cannot destroy the Constitution's core.

The turning point came through expansive reading of Article 21. In Maneka Gandhi v. Union of India (1978), the Court held that 'procedure established by law' must be fair, just, and reasonable — opening the door to substantive due process. Subsequent rulings used this to absorb DPSP-backed values: Olga Tellis v. Bombay Municipal Corporation (1985) recognised the right to livelihood; Unni Krishnan v. State of AP (1993) the right to education; MC Mehta cases established the right to a clean environment.

The doctrinal tool is harmonious construction: where Fundamental Rights and DPSPs appear to conflict, courts read them together to advance constitutional goals. The Supreme Court described them as 'two wheels of a chariot' in Minerva Mills (1980), prohibiting Parliament from giving DPSPs absolute supremacy over Part III. The equilibrium — neither part can destroy the other — is itself a basic structure element.