The doctrine of severability is a guardian of our fundamental rights, suppose if any of the provisions in an act/statue is contrary to our fundamental rights then that provision only would consider being void and it is not the whole act that becomes void.
The nature of the provision is material, suppose if any of the provision or any sub section is void due to its unconstitutional nature and if due to this the entire section adopts a different meaning such that the scheme of the whole section in the act changes, then the whole section must be declared un constitutional. The doctrine of severability depends on validity of provisions in an act and the effect of those provisions on the whole act itself.
This position of the law is proved in various cases
A.K. Gopalan v. State 0/ Madras, A.I.R. 1950 S.c. 27 where section 14 of prevention detention act was found out to be in violation of Article 14 of the constitution. It was held by the Apex court that it is section 14 of the act which is to be struck down not the act as a whole. It was also held that the omission of section 14 of the act will not change the object of the act and hence it is severable.
R.M.D.C. v.Union of India, Where the provisions of the act are so mixed together i.e the invalid portion and the valid portion such that it would not possible to separate them, then the act as whole would deemed to be void.
The doctrine of severability is necessary to protect the validity of the act as a whole without which an entire act would become void due to invalidity of one provision of the act. Now it is upto the courts to decide the question related to the effects of invalid provisions on the scheme of the act and accordingly adjudicate the question of declaring the validity of the act as a whole and various provisions of the act.