Discuss the principles and philosophy underlying the doctrine of 'basic structure' as enunciated in Keshvananda Bharti's case. Which features of the Constitution have so far been declared as part of its basic structure by the Supreme Court?

Prior to Kesavananda Bharatis case there was no effective limitation on the power of Parliament to amend any part of the Constitution. There was nothing that prevented Parliament from taking away a citizen’s right to freedom of speech or his religious freedom too. The repeated amendments raised a doubt: was there any inherent or implied limitation on the amending power of Parliament? This was solved in Kesavanada Bharti Case whereby Supreme Court laid down the Basic Structure Doctrine.  
Thus, the basic principle and philosophy is that there are certain provisions in the Constitution which cannot be amended even by the following prescribed procedure under Article 368. However, what is included in this doctrine was not defined in that case. From various judgments, what emerged is a plethora of subjects which are now considered to be a part of this doctrine. Some of the important subjects include:

• Supremacy of the Constitution
• Republican and democratic form of government; Parliamentary system ; Free and Fair elections
• Federal and Secular character of the Constitution
• Separation of powers between the legislature, executive and the judiciary
• Unity, Integrity and sovereignty of the country
• Essential features of individual freedoms including religious freedom
• Equality of status and opportunity and mandate to create a welfare state.
• Independent Judiciary and Power of Judicial Review
• Rule of law.

The Basic structure has helped to retain the basic ideals of the constitution but has been also a subject of criticism and alleged to be anti-democratic character and an instrument of judicial hegemony.