In brief: With the poorest thing that can be thought — pure being. And with a resolve of which Hegel himself says that it may be taken for an arbitrary act. The beginning does not carry the system; it is carried by it. The Groundlines do not make this beginning: they are a part, and presuppose their concept as given.
The problem
A science that wants to ground everything may not itself rest on an ungrounded assumption. So its first proposition may presuppose nothing. But a proposition that presupposes nothing has no content either — and from nothing, nothing follows. Hegel holds the problem to be new, not eternal:
It is only in recent times that consciousness has arisen of there being a difficulty in finding a beginning in philosophy.
Science of Logic I, „With what must the beginning of science be made?“
Formerly one asked after the principle — water, the One, substance, monad — and that was a question about content, not about beginning. Only when method itself became the theme did it emerge that the two must coincide: the first in thinking and the first of what is thought.
Hegel’s answer: pure being
If the beginning may presuppose nothing, it can contain no determination either — for every determination would already be a difference and hence a mediation. Exactly one thing remains:
it may thus presuppose nothing, must be mediated by nothing nor have a ground; rather it is itself to be the ground of the entire science. […] The beginning is therefore pure being.
Science of Logic I
That is no claim about the world. It is the observation that at the beginning there stands nothing one could dispute — and precisely in that lies its freedom from presupposition. That anything at all follows from it is the scandal of the Logic: being without any further determination is not to be distinguished from nothing. The beginning moves because it is empty.
The resolve
A remainder is left, and Hegel does not conceal it. That one begins with pure thinking follows from nothing:
There is present only the resolve, which may also be regarded as arbitrary, namely that one wills to consider thinking as such.
Science of Logic I
This sentence is readily read as the admission of a failure. It is rather the opposite: a warning against expecting more of the beginning than it can carry. One cannot bring thinking to begin by an argument; one can only begin.
Why the beginning is not the foundation
The real resolution lies elsewhere. Hegel’s system is no building resting on a cornerstone, but a movement that comes back to its point of departure:
What is essential for science is not so much that a pure immediate be the beginning, but that the whole of it is a circuit within itself, in which the first is also the last and the last also the first.
Science of Logic I
In this the beginning loses its one-sidedness — it becomes, retrospectively, what it claimed to be:
Through this progression, then, the beginning loses the one-sidedness it has in this determinacy of being something immediate and abstract in general; it becomes something mediated, and thereby the line of scientific advance makes itself into a circle.
Science of Logic I
The beginning is therefore grounded not before the science but through it. That is circular — and Hegel does not dispute it but claims that every science which grounds itself must have this shape. Whoever holds this to be a fallacy must show how it could be otherwise; whoever holds it to be a triviality must explain why the search for the ultimate foundation has never stopped.
And the Groundlines?
They precisely do not make this beginning. The second paragraph says with remarkable openness that the science of right does not ground its own basic concept:
As a part it has a determinate starting point, which is the result and the truth of what precedes […]. The concept of right therefore falls, as regards its coming to be, outside the science of right; its deduction is here presupposed, and it is to be taken up as given.
Groundlines, § 2
For reading this has a very practical consequence: one need not have read the Logic in order to begin at § 1. The Groundlines begin where a reader can attach something to the word „freedom“, and work from there. At length: Do I need to know the Logic?
What Hegel excludes in § 2 Remark, by contrast, is the convenient substitute for a beginning: starting with a definition. Definitions abstract from existing representations and measure themselves against them — and so become false as soon as the representations are false. His example has not been bettered: in Roman law no definition of the human being would have been possible, „for the slave could not be subsumed under it“. It is not the definition that would have settled the case — the case would have refuted the definition.
Sources. Science of Logic I, First Book, Introduction: „With what must the beginning of science be made?“ (GW 21, 53–68; TWA 5, 65–79). – Encyclopaedia (1830) §§ 1–18 on the relation of part and whole, §§ 61 ff. on immediate knowing. – Groundlines § 2 and Remark; the warning omnis definitio in iure civili periculosa comes from Dig. 50, 17, 202 (Iavolenus). – See also Science of Logic and Immediacy and Mediation in the glossary. – Literature. Robb Dunphy, „Hegel and the Problem of Beginning“, Hegel Bulletin 42/3 (2021); Dieter Henrich, „Anfang und Methode der Logik“, in: Hegel im Kontext, Frankfurt a. M. 1971; Kevin Thompson, Hegel’s Theory of Normativity, Evanston 2019.