In object-oriented programming and your example, @obj1 is not being
"passed" to perform_sanity_check, it is the "receiver object," the
object being "asked" to execute its perform_sanity_check method. In a
way, you are sending a message to the object @obj, requesting that it
execute one of its methods.
Does @obj1 have access to @obj1's attributes? That depends on the
context. If @obj1.perform_sanity_check appears in the implementation
of one of @obj1's instance methods, inside the @obj1's class
definition, then @obj1 can access the instance variables using
self.instance_variable or @instance_variable. Otherwise, the instance
variables aren't directly accessible (Ruby encapsules object
impelementation). They may be accessible through accessor methods,
methods that may be defined in @obj1's class definition provided to
(indirectly) access them.
See: "The Ruby Programming Language" by Flanagan & Matsumoto, Chapter
7: Classes and Modules.