Petty understood risk. In the Quantulumcunque concerning Money (1682) 
he writes (cut-and-paste from the late Rod Hay's website):

Qu. 32. What do you think of our Laws for limiting Interest?
     Answ. The same as limiting the Exportation of Money; and there 
may be as well Laws for limiting Exchange also: For
interest always carrieth with it an Ensurance praemium, which is very 
casual, besides that of Forbearance: For Instance, in
Ireland there was a time when Land (the highest Security) was sold 
for 2 years Purchase: it was then naturally just to take 20,
30, or 40 per cent Interest; whereas there the Law allows but 10. And 
since that time, Land being risen to 12 Years purchase,
responsible Men will not give above 8. And insolent Men will offer 
cent per cent notwithstanding the Law. Again, suppose a Man
hath 100 l. of Land, worth 20 Years Purchase, and another 100 l. in 
Houses worth 12 Years Purchase; and an other 100 l. in
Shipping, worth 2 Years Purchase; and another in Horses, worth 6 
Months Purchase; Is it not manifest he must have a greater Yearly 
praemium for lending his House than his Land, his Ship than his 
House, and his Horse than his Ship? For if his Horse be worth 100 l. 
he cannot hire out for than 10s per diem, whereas the Land will not 
yield a Groat for the same time; and these Hires are the same with Interest.

There is, however, more than scientific detachment in Petty's 
statements.  He had played a very sinister and active role in Irish 
landgrabbing himself, and all his talk about risk discounting is 
sheer hypocrisy.  The British took their land and then they 
complained that it was unproductive, because of Irish 
resistance.  Petty then plays scientist and adjusts the value 
of  Irish land to take into account such inconveniences. Read 
Alessandro Roncaglia's book on Petty to see what I mean.

How doth the little crocodile.

Nicholas Theocarakis