Origin of the doctrine
The origin of this doctrine lies in Canada through the case named Cushing Vs. Dupuy (Important judgment because it paved the roots for Ancillary and Incidental encroachment) and slowly made its way to India and is firmly supported by Article 246 and Seventh Schedule (The Constitution of India has divided the extent of legislative powers between the Centre and states by way of the Seventh Schedule. This schedule consists of, Union, State and Concurrent Lists) of Indian Constitution. This doctrine has become the basis of many Supreme Court Judgements.

Meaning of the Doctrine
To disintegrate the doctrine to its molecular meanings, Pith denotes true nature or essence of something and Substance means the most important or essential part of something. The definition of this doctrine states, within their respective spheres the state and the union legislatures are made supreme, they should not encroach upon the sphere demarcated for the other.

Doctrine of Pith and Substance is applied when legislation made by of the legislatures is challenged or trespassed by other legislatures. This doctrine says that when there is a question of determining whether a particular law relates to a particular subject the court looks to the substance of the matter. If the substance of the matter lies within one of the 3 lists, then the incidental encroachment by law on another lists, does not make it invalid because they are said to be intra vires.

Features of Doctrine

1. This doctrine comes into application when subject matter between 2 lists seems to be conflicting.
2. The powers of legislature will be strictly limited if every law is termed invalid on the grounds that it is encroaching upon other laws.
3. The doctrine pulls out the true nature and character of the matter to bifurcate it into its appropriate list.

Landmark Judgments

1. State of Bombay Vs. F.N. Balsara, was a case in which Bombay Prohibition Act was challenged on the grounds that the prohibition of liquor on the borders was a matter of Central Government. The act was held valid by the court because it was in its pith and substance and fell under the State List though it was impacting the import of liquor.
    
2. Prafulla Kumar Mukherjee v. Bank of Khulna, was the case in which Bengal Money Lenders Act, was challenged on the grounds that it limited the rate of interest and the amount recoverable by a money lender on any loan. It was argued that promissory notes were a central matter and not a state matter. It was held by the Privy Council that the act was in piths and substance and law with respect to 'money lending and money lenders' was a state matter and was valid even if incidental encroachment upon 'Promissory notes' which is Central matter was taking place.

Analysis
This doctrine has been an important aspect in various Court judgements. Every time question of overlapping or encroachment comes rises the doctrine of Pith and substance gives a clarity towards the subject matter and in which list does it belong to. This doctrine is observed to provide an ease to the otherwise rigid federal structure which distributes the powers between Central and State Government.

If it was so that every legislation was to be termed invalid for encroaching upon the fields of other legislatures then the powers would not hold any weight and the purpose of such powers being bifurcated would be lost because of extreme rigidity.

Conclusion
To conclude, Pith and Substance denotes the true nature of law. The doctrine emphasis on the real subject matter and not its effects on other fields/laws.