ruby - Understanding a Boolean conjunction statement in Rails method -

this might trivial point, don't understand , bothers me. going through ruby on rails tutorial michael hartl, there method definition :

 def create     user = user.find_by(email: params[:session][:email].downcase)     if user && user.authenticate(params[:session][:password])       log_in user       redirect_to user     else       flash.now[:danger] = "invalid email/password combination"       render 'new'     end   end 

what don't understand why conjunction

user && user.authenticate(params[:session][:password])    

is neccessary. surely, there no case in user.authenticate return "true" if user object empty, , hence, nil. matter of style, or 'user &&' subtle in expression?

trying part: user.authenticate(params[:session][:password]) without first knowing if object exists throw error. need make sure have user object before try run .authenticate on it. make sense?