calling method from class methods

When calling another methods from class method , the method must be another class method. Why ? Because it doesn't make sense to call an instance method from a class method. What if there is no object instantiated.


def self.classify(number)
    raise RuntimeError if number <= 0
    sum = sum_of_factors(number)
    if sum == number
      "perfect"
    elsif sum > number
      "abundant"
    elsif sum < number
      "deficient"
    end
  end
 

  def self.sum_of_factors(number)
    factors = (1...number).select { |n| number % n == 0 }
    sum_of_factors = factors.reduce(:+)
  end