Hi there,

I've got a text excerpt here and I'm wondering whether the modal "must" has epistemic or deontic meaning. What do you think? (it's from the end of the 17th century)

For understanding of this, let us suppose the quadrat
space E, to be filled with a piece of Lead or Iron. I
say then it must go down to I; and the reason is, because
the quadrat foot of Water I, is more pressed then the
quadrat foot of Water K. To illustrat this, let us suppose
that each quadrat foot of this Water weights a pound,
and that the heavy body existing in E, weights two
pound. If this be, the foot of Water I, must yeeld, seeing
it is more prest then K: upon the same account must
the Water N yeeld, and give way to the Stone, seeing
it is more prest then R. For according to the twelfth
Theorem, (^There cannot be unequal Pressure upon a surface,
unless motion follow^) .
   For understanding the second part, let us suppose the
space R, to be filled with a piece of Cork, that is             #
specifically
or naturally lighter then Water. I say then, it
must ascend to the top B; and the reason is, because the
quadrat foot of Water K, is more prest upward, then the
quadrat foot of Water I, or L is: but this cannot be in
Fluid bodies, unless motion follow thereupon. I say, it
is more prest up, because R being lighter then N, or S,
it must press with greater force upon K, then S can do
upon L, or N upon I. It is still to be remembred,
(^That Fluids presseth with as much strength upward, as         #
downward^) ,
according to the sixth Theorem; and that an Horizontal
surface doth as really suffer unequal Pressure from
below, as from above.

In my opinion it doesn't express obligation but rather an assumption of the speaker. But is it really epistemic?