[StevePool (OP)]

The alphabet and our numerals once coincided, but changes over the centuries to either Greek letters or Arabic numerals mean that only four obvious parallels now survive:

Α       1
Β       2
Γ       3
Δ       4
Ε       5
Ζ       6
Η η     7
Θ       8
Ι       9
Κ       X
 
So in fourth place, Δ and 4 both depict sails. Moreover, the Old Irish name of the fourth letter in the ogam alphabet was sail.

In seventh place, η and 7 both depict horse’s head and neck. The other form of ēta depicts a chariot seen from above: Η.

In eighth place, Θ and 8 both depict breasts. (Note also the link between Albanian for eight, tetë, and Welsh teth, a teat; Italian tetta, a breast; Lithuanian teta, an aunt; and Armenian tit, a teat.)

Finally, in tenth place, Κ and X (Roman 10) look almost the same. Many languages have related words for hand and ten, and therefore Latin captō, to grasp; Greek kaptō, to snatch; Albanian kap, to grab; Hittite kappuwa-, to count; and Persian kp-, to hit, are all related to the tenth letter, kappa. In the same way, Greek dekomai, to take; Greek dektēr, a collector; and Tocharian B täk-, to fetch and to touch, are all related to Cornish dec, Latin decem, Greek deka and other words meaning ten.

[Bored chemist]

Moreover, the Old Irish name of the fourth letter in the ogam alphabet was sail.

Which seems to mean "fat" in modern Irish, and willow tree in Old irish.
So it's not got anything to do with sails (and more to do with the Latin "Salix", meaning willow.

In eighth place, Θ and 8 both depict breasts.

Looks a bit Freudian to me.
The original "8" didn't look like that
https://en.wikipedia.org/wiki/8#Glyph

Ditto Theta.
https://en.wikipedia.org/wiki/Greek_alphabet#Origins

[StevePool (OP)]

You have a touching faith in Wikipedia.
But the point of Day's book is that the alphabet didn't come from Phoenicians -- so it's pointless to cite any supposed Phoenician version of theta! And he argues that our numerals didn't come from India -- so it's equally pointless to cite any supposed version of 8! Try reading the book before criticising it ...

[Bored chemist]

But the point of Day's book

What book?

Your faith in our ability to read minds is not so much touching, as silly.