= Solution
The <dual projective space> is $\mathbb P((\mathbb R^3)^*)$. A <projective line> with coefficients $[\alpha:\beta:\gamma]$ becomes a <projective point> in this dual plane; a <projective point> $[a:b:c]$ becomes the dual <projective line> $a\alpha+b\beta+c\gamma=0$.
The <point-line duality> reverses incidence:
$$
\boxed{P\in l\quad\Longleftrightarrow\quad l^*\in P^*.}
$$
Thus \b[the dual statement is that the dual point $l^*$ lies on the dual line $P^*$]. A join of <projective points> becomes an intersection of dual <projective lines>, and the converse holds too. In particular, <collinear> <projective points> correspond to concurrent dual <projective lines>.
The dual of <Desargues's theorem> is its converse: \b[if the intersections of the three pairs of corresponding sides are <collinear>, then the three lines joining corresponding vertices are concurrent]. In the same nondegenerate setting, <perspectivity from a line> therefore implies <perspectivity from a point>.
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