In The Principal of Reason (1957) Heidegger links reason to being, so whereas Descartes says:
“I now know that even bodies are not strictly perceived by the senses or the faculty of imagination but by the intellect alone and that this perception derives not from their being touched or seen but from their being understood” (Descartes 22).
If I hear this right, Heidegger goes a step further and doesn't just say rationalisation makes the being of the wax understood; he says rationalisation makes the being of the wax. Intelligibility not only makes the thing understood, it makes the thing.
Being leads to beings and reason, by rationalising, also leads to beings.
Here is a glimpse of how Heidegger connects reason and being in The Principal of Reason:
[Finally] we hear the principle of reason in a different tonality.
Instead of "Nothing is without reason," it now sounds like this:
"Nothing is without reason." The pitch has shifted from the
"nothing" to the "is" and from the "without" to the "reason". The word
"is" in one fashion or another invariably names being. This shift in
pitch lets us hear an accord between being and reason. Heard in the
new tonality, the principle of reason says that to being there belongs
something like ground/reason. The principal now speaks of being. What
the principal now says, however, easily falls pray to a
misinterpretation. "Ground/reason belongs to being"—one might be
inclined to understand this in the sense of "being has a reason," that
is, "being is grounded." The popularly understood and presumably valid
principium rationis never speaks of this. According to the principal of reason, only beings are ever grounded. On the contrary,
"ground/reason belongs to being" is tantamount to saying: being qua
being grounds. Consequently, only beings ever have their grounds.
The new tonality reveals the principle of reason as a principle of
being. Correspondingly, if we now discuss the principle in the new
tonality, we move in the realm of what one can, with a general term,
call the "question of being." (p.50-51)
Hence, there is a peculiar state of affairs with the "is" and "being".
In order to respond to it, we articulate what the principle of reason
says as a principle of being as follows: Being and ground/reason: the
same. Being: the abyss. As we remarked, to say "being" "is"
ground/reason is inadmissible. This way of speaking, which is
virtually unavoidable, does not apply to "being"; it does not hit upon
its proper character. (p.51)
The principle of reason is not only a principle in the sense of a
supreme fundamental principle. The principle [Satz] of reason is a
Satz in the eminent sense of being a leap. [The German] language knows the form of speech: With a vault, that is, with a sudden leap
he was out the door. The principle of reason is a vault into the
essence of being in the sense of such a leap. We really ought not any
longer say the principle of reason is a principle of being; rather, we
should say that the principle of reason is a leap into being qua
being, that is, qua ground/reason. (p.53)
