Marx and Engels said:



  It is an unavoidable phenomenon, well established in the course of
  development, that people from the ruling class also join the
  proletariat and supply it with educated elements. This we have already
  clearly stated in the Manifesto. Here, however, two remarks are to be
  made:

  First, such people, in order to be useful to the proletarian movement,
  must bring with them really educated elements. This, however, is not
  the case with the great majority of German bourgeois converts. . .
  .They are completely deficient in real, factual, or theoretical
  material. Instead, there are efforts to bring superficial socialist
  ideas into harmony with the various theoretical viewpoints which the
  gentlemen from the universities, or from wherever, bring with them,
  and among whom one is more confused than the other, thanks to the
  process of decomposition in which German philosophy finds itself
  today. Instead of first studying the new science [scientific
  socialism] thoroughly, everyone relies rather on the viewpoint he
  brought with him, makes a short cut toward it with his own private
  science, and immediately steps forth with pretensions of wanting to
  teach it. Hence, there are among those gentlemen as many viewpoints as
  there are heads; instead of clarifying anything, they only produce
  arrant confusion -- fortunately, almost always only among themselves.
  Such educated elements, whose guiding principle is to teach what they
  have not learned, the party can well dispense with.

  Second, when such people from other classes join the proletarian
  movement, the first demand upon them must be that they do not bring
  with them any remnants of bourgeois, petty-bourgeois, etc.,
  prejudices, but that they irreversibly assimilate the proletarian
  viewpoint. But those gentlemen, as has been shown, adhere
  overwhelmingly to petty-bourgeois conceptions. In so petty-bourgeois a
  country as Germany, such conceptions certainly have their
  justification, but only outside the Social-Democratic Labor party. If
  the gentlemen want to build a social-democratic petty-bourgeois party,
  they have a full right to do so; one could then negotiate with them,
  conclude agreements, etc., according to circumstances. But in a labor
  party, they are a falsifying element. If there are grounds which
  necessitates tolerating them, it is a duty only to tolerate them, to
  allow them no influence in party leadership, and to keep in mind that
  a break with them is only a matter of time.

  "STRATEGY AND TACTICS OF THE CLASS STRUGGLE" by KARL MARX and
  FREDERICK ENGELS September 17-18 1879

  A Private Circulation Letter from Marx and Engels (First drafted by
  Engels) to Germany's Social-Democratic leadership