Introduction:
The UPSC Law Optional subject is a highly competitive and prestigious examination conducted by the Union Public Service Commission (UPSC) in India. Aspirants who choose law as their optional subject need to have a comprehensive understanding of various legal doctrines. Here are some important legal doctrines that candidates should be familiar with:

1. Doctrine of Stare Decisis:
- Stare Decisis is a Latin term that means "to stand by decided cases".
- This doctrine is based on the principle that once a legal principle or decision has been established by a higher court, lower courts are bound to follow it.
- It provides consistency and certainty in the legal system and ensures that similar cases are decided in a similar manner.

2. Doctrine of Res Judicata:
- Res Judicata means "a matter already judged".
- This doctrine prevents the same matter from being re-litigated between the same parties.
- It promotes finality and certainty in legal proceedings and avoids multiple lawsuits on the same issue.

3. Doctrine of Precedent:
- Precedent refers to a legal decision or principle that serves as a guide for future similar cases.
- The doctrine of precedent allows for consistency and predictability in the legal system.
- Lower courts are generally bound by the decisions of higher courts, creating a hierarchy of precedents.

4. Doctrine of Separation of Powers:
- The doctrine of separation of powers is a fundamental principle of constitutional law.
- It divides the powers of the government into three branches: the legislature, the executive, and the judiciary.
- This doctrine ensures a system of checks and balances, preventing any one branch from becoming too powerful.

5. Doctrine of Fundamental Rights:
- Fundamental rights are basic human rights guaranteed by the Constitution.
- The doctrine of fundamental rights provides individuals with certain protections and freedoms.
- It includes rights such as the right to equality, right to freedom of speech, right to life and personal liberty, etc.

Conclusion:
Familiarity with these legal doctrines is essential for candidates appearing for the UPSC Law Optional subject. These doctrines form the basis of the Indian legal system and are crucial for understanding the principles of law and their application. Aspirants should study these doctrines thoroughly and develop a clear understanding of their significance in order to excel in the examination.