1. At the beginning of the game the forces stand in
   equilibrium.
2. Correct play on both sides maintains this equilibrium and leads
   to a drawn game.
3. Therefore a player can win only as a consequence of an error
   made by the opponent. (There is no such thing as a winning
   move.)
4. As long as the equilibrium is maintained, an attack, however
   skilful, cannot succeed against correct defence. Such a defence
   will eventually necessitate the withdrawal and regrouping of the
   attacking pieces and te attacker will then inevitably suffer
   disadvantage.
5. Therefore a player should not attack until he already has an
   advantage, caused by the opponent’s error, that justifies the
   decision to attack.
6. At the beginning of the game a player should not at once seek
   to attack. Instead, a player should seek to disturb the equilibrium
   in his favour by inducing the opponent to make an error – a
   preliminary before attacking.
7. When a sufficient advantage has been obtained, a player must
   attack or the advantage will be dissipated.”

From David HOOPER, Steinitz’ Theory, British Chess
Magazine Vol. 104, p.370 Sept 1984.

 References:

• Steinitz’ International Chess Magazine
• Steinitz’ Modern Chess Instructor
• Lasker’s Manual of Chess
• Euwe’s The Development of Chess Style

Notes

1. The theory is the antithesis of the previously held view, that
   inspired play might of itself create a winning position;
   rather, (3) winning positions can come about only through errors by
   the opponent. Creativity and ingenuity still have a role, in the
   setting of problems for the opponent that they may fail to
   solve.
2. The point (4), that, if a defender played correctly, then any
   attack commencing from an equal position can be refuted, was first
   indicated by Louis Paulsen. He declared (unfashionably) that all
   gambits can be defended, and practised what he preached; in this
   way Paulsen was the progenitor of the fully formed theory of
   Steinitz. Defensive play has its own principles:
   The defending side must be prepared to defend and make
   concessions, e.g making a weakening pawn move. However, the
   defender should avoid making concessions until forced, and then
   should make only the minimum concession necessary to meet the
   threats.

    

3. The last point (7) is attributed by Lasker to Steinitz; DH
   cautions that he was unable to find such a statement in Steinitz’
   works. Lasker’s version goes on to say:
   The right to attack belongs only to that side which has a
   positional advantage, and this is not only a right, but also a
   duty, otherwise there is the risk of losing the advantage. The
   attack is to be directed against the weakest spot in the opposing
   position.
4. DH was prompted to write this article after reading Purdy’s
   “The Great Steinitz Hoax”, Chess Player’s Quarterly, 1978
   [which is, according to BCM, reprinted in No.2 of Lasker and
   his Contemporaries], in which Purdy apparently argues that
   Lasker was more the author of the theory than Steinitz.

   Purdy and Hooper disagree on what constitutes the theory.

5. Lasker certainly was the principal pedagogue and advocate of
   (his version of) Steinitz’ theory through his Manual.
   Lasker’s presentation was coloured to some extent by the principles
   of his own general philosophy of Struggle (Work, Economy
   and Justice); Lasker also offered extensions to the Steinitz
   theory, viz. principles of Cooperation and of Justice (in the
   sphere of chess).
6. The influence of the theory is so substantial that instances
   need hardly be found, but a couple interested me recently:
   1. Jon Speelman’s introduction to his Best Games
      1970-1980 collection, where he refers to the “story” style of
      annotation – where was the crucial error by the loser? This of
      course assumes that the loser must have made at least one mistake, and Speelman quite explicitly
      signs up to the theory that there is Justice in chess, and that to
      lose you must have made a mistake – or two, really, and maybe three
      if you are White.
   2. An annotation in Chess Monthly where the GM
      reluctantly admitted that despite a series of brilliant moves by
      Black, White was at no disadvantage. This is the corollary of the
      same point; in a balanced position, even brilliant play, correctly
      met, will lead only to another balanced position.
7. What is the significance of Steinitz’ theory for the club
   player? Different aspects have different importances.
   1. Defensive play may be so poor that an attack undertaken with
      insufficient means might well succeed in practice.
   2. So, defensive play, as modelled by Paulsen and Steinitz, must
      become a strength of yours.
   3. Moreover, while winning combinations may theoretically be
      available principally (if not
      only) in advantageous positions, in practice tactical
      opportunities and especially oversights can appear at any time, so
      at each move you should routinely check for combinational blows for
      each side.
   4. It may not be so important to decide who has the advantage as
      to decide what to do, and then do it well!
      “The question that matters to you in actual play is simply,
      ‘What is my best move?’, and if you can decide without being sure
      who has the theoretical advantage, so much the better” —
      PURDY