5.452 The introduction of a new device in the symbolism of logic must always be an event of great consequences. No new device may be introduced into logic – with, so to speak, a wholly innocent face – in brackets or in a footnote.

(Thus in the Principia Mathematica of Russell and Whitehead there appear definitions and basic laws in words. Why suddenly words here? This would need a justification. This is missing and must be missing, since the procedure is actually forbidden.)

If, however, the introduction of a new device has proved necessary in one place, then one must ask oneself straightaway: Where must this device now always be used? Its place in logic must now be made clear.

All sounds fair enough. Black (p. 266): “An example of what Wittgenstein has in mind is the ‘primitive proposition’ *1.1: ‘Anything implied by a true elementary proposition is true’ (Principia, vol. 1, p. 94).”