The statement, 'The structure of the Indian Constitution is federal but its soul is unitary,' encapsulates the unique nature of India's constitutional framework. India is often described as a 'quasi-federal' state or a 'federal system with a strong unitary bias' by constitutional experts like K.C. Wheare. This implies that while the Constitution exhibits several features characteristic of a federal system, it also possesses strong centralizing tendencies that make it lean towards a unitary form of government, especially during emergencies or specific circumstances.

Federal Features (Structural Aspects): These are the elements that give the Indian Constitution its federal 'structure':

1. Dual Polity: The Constitution establishes a dual government, with the Union at the Centre and states at the periphery, each exercising powers in its own sphere.
2. Written Constitution: India has a comprehensive, written constitution that clearly defines the powers and functions of both the Union and state governments.
3. Supremacy of the Constitution: The Constitution is the supreme law of the land, and both the Centre and states must operate within its framework.
4. Division of Powers: The Seventh Schedule of the Constitution divides legislative powers between the Union and states through the Union List, State List, and Concurrent List.
5. Independent Judiciary: An independent judiciary, headed by the Supreme Court, acts as the interpreter and guardian of the Constitution, resolving disputes between the Centre and states.
6. Bicameralism: The Parliament consists of two houses: the Lok Sabha (representing the people) and the Rajya Sabha (representing the states), reflecting the federal principle.
7. Rigidity of the Constitution: Certain provisions of the Constitution, particularly those affecting the federal structure, require a special majority for amendment, ensuring that states' rights are not easily overridden.

Unitary Features (Soul/Spirit Aspects): These are the elements that give the Indian Constitution its unitary 'soul' or 'bias':

1. Strong Centre: The Union List contains more important subjects, and the Centre has residuary powers. States are financially dependent on the Centre, and the Centre can legislate on State List subjects under certain conditions.
2. Single Constitution: Unlike typical federations (e.g., USA), India has a single constitution for both the Union and states, except for Jammu and Kashmir (which had its own constitution until 2019).
3. Single Citizenship: All Indians possess a single citizenship, regardless of their domicile, fostering national unity.
4. Integrated Judiciary: India has a unified and integrated judicial system, with the Supreme Court at the apex, followed by High Courts and subordinate courts. This contrasts with dual judicial systems in true federations.
5. Appointment of Governors: The Governor of a state is appointed by the President and acts as an agent of the Centre, often leading to central interference in state affairs.
6. All-India Services: Services like IAS, IPS, and IFS are recruited and controlled by the Centre but serve in states, ensuring uniformity in administration and loyalty to the Centre.
7. Emergency Provisions: During a national, state, or financial emergency, the Centre can assume vast powers, transforming the federal system into a unitary one without formal amendment.
8. Parliament's Power to Alter State Boundaries: Parliament can unilaterally change the name, area, or boundaries of states without their consent.
9. Veto Over State Bills: The Governor can reserve certain state bills for the President's consideration, giving the Centre a veto power over state legislation.
10. Unequal Representation in Rajya Sabha: States are not equally represented in the Rajya Sabha, which is a departure from the federal principle found in countries like the USA.

Conclusion: India's constitutional design is a pragmatic response to its unique historical context, diverse population, and the imperative of national unity and integrity. The framers deliberately created a strong Centre to prevent Balkanization and ensure stability, while also granting states significant autonomy. This blend of federal and unitary features allows for flexibility, enabling the system to adapt to various situations. Thus, India's federalism is often described as 'cooperative federalism' or 'bargaining federalism,' where the unitary spirit ensures national cohesion without completely undermining the federal structure.