There are plenty of ways that a tax can raise an amount from individuals that is proportional to the revenue which they enjoy under the protection of the state

There are plenty of ways that a tax can raise an amount from individuals that is proportional to the revenue which they enjoy under the protection of the state besides an income tax. Basically any tax on any broadly consumed commodity will do that, because its consumption will be proportional to overall-consumption, and by extension, revenue. An income tax did not exist in Smith's time, and had it been proposed in that period, it's extremely unlikely he would have favored it given 1. his admonishments against expansive government that intrudes upon the private lives of citizens, and 2. the need for an expansive tax enforcement apparatus, that invades the inner sanctums of the private lives of citizens, to enforce an income tax.