What is the need for an Explanatory theory?
What is it about?

It's needed because of the question:
How to choose the right Grammar?

• naive: the simplest? But what is simple?
• Fewer rules?
• Fewer syntactical categories?
• Only rules with a particular form?

=> No a priori idea
=> further research needed

Explanatory theory:

• method of choosing the right grammar
• includes hypothesis about general properties of human language => Universal Grammar