Salesforce Platform Developer I Certification Guide

Private

This is the default definition. If you don't add an access modifier to your class, variable, or method, then Apex will automatically add the private modifier (you don't see it, but it is there, believe me). The variables and methods defined as private are only usable in the defining class (not in another class, and not in a subclass, and not even in a Visualforce page, if you write Visualforce controllers or extensions).