Penrose (The Road to Reality, Section 3.2) describes Dedekind as defining real numbers via a "knife-edge" cut in the size-ordered sequence of rationals, separating them into two sets; where a cut does not fall on a rational, it must therefore fall on a kind of number which we call real.
I find two flaws in this. In what follows I shall use the term "two sets of rationals" to indicate a division of all rationals into two sets, ordered by size.
The first flaw appears to be circular reasoning. On what basis does Dedekind allow that a gap between the two sets of rationals has any meaning? It seems to me that he implicitly assumes the existence of reals, for which he must already have a definition in order to assume anything about them. Assuming your result is not valid logic.
Next, consider a convergent series and its termination, such as 1/2 + 1/4 + 1/8 ... = 2. The shortfall in the series gets smaller and smaller (in this example, at any point in the series it is equal to the latest term in the series). Yet, in the calculus at least, we are happy to accept that in the limit, the shortfall vanishes and the equality expressed above is valid; this is how we justify a continuous derivative of a continuous curve, for example. Now apply this to the rational number line. As more rationals are included, the gaps between the two sets of rationals shrinks. In the limit, by the reasoning of the calculus the gap also vanishes, as do all gaps where other cuts might fall, and the rational number line becomes continuous. This appears to undermine the assumption of a gap between two sets of rationals.
But I am somewhat confused by this question on Proof on density of rationals and irrationals in R via dedekind cut, which appears to treat a Dedekind cut as a much blunter instrument than Penrose does, by talking of "infinitely many rationals that are greater than other rational in our dedekind cut".
Who offers the flawed argument; Dedekind, Penrose or me, and where is the flaw?