Synthesized answer
James views monism as an hypothesis because radical empiricism, which he espouses, regards its most assured conclusions concerning matters of fact as hypotheses subject to modification by future experience [1]. Radical empiricism treats the doctrine of monism itself as an hypothesis, differentiating itself from half-way empiricism that dogmatically affirms monism [1].
The passages indicate that _prima facie_ the world appears as a pluralism, and while higher thinking aims to discover more unity, absolute unity remains undiscovered and a _Grenzbegriff_ [1]. The essayist associates a radical empiricist with taking the notion that pluralism is the permanent form of the world as their hypothesis [4]. The passages do not further elaborate on the specific reasons why James views monism as a hypothesis beyond its classification within his framework of radical empiricism and the perceived nature of the world as pluralistic.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
of _radical empiricism_, in spite of the fact that such brief nicknames are nowhere more misleading than in philosophy. I say 'empiricism,' because it is contented to regard its most assured conclusions concerning matters of fact as hypotheses liable to modification in the course of future experience; and I say 'radical,' because it treats the doctrine of monism itself as an hypothesis, and, {viii} unlike so much of the half-way empiricism that is current under the name of positivism or agnosticism or scientific naturalism, it does not dogmatically affirm monism as something with…
eternal element thereof. There is no possible point of view from which the world can appear an absolutely single fact. Real possibilities, real indeterminations, real beginnings, real ends, real evil, real crises, catastrophes, and escapes, a real God, and a real moral life, just as common-sense conceives these things, may remain in empiricism as conceptions which that philosophy gives up the attempt either to 'overcome' or to reinterpret in monistic form. Many of my professionally trained _confrères_ will smile at the irrationalism of this view, and at the artlessness of my essays…
rket-place. But it is just on this matter of the market-place that I think the utility of such essays as mine may turn. If {xii} religious hypotheses about the universe be in order at all, then the active faiths of individuals in them, freely expressing themselves in life, are the experimental tests by which they are verified, and the only means by which their truth or falsehood can be wrought out. The truest scientific hypothesis is that which, as we say, 'works' best; and it can be no otherwise with religious hypotheses. Religious history proves that one hypothesis after another has…
; and there may be in the whole universe no one point of view extant from which this would not be found to be the case. "Reason," as a gifted writer says, "is {ix} but one item in the mystery; and behind the proudest consciousness that ever reigned, reason and wonder blushed face to face. The inevitable stales, while doubt and hope are sisters. Not unfortunately the universe is wild,--game-flavored as a hawk's wing. Nature is miracle all; the same returns not save to bring the different. The slow round of the engraver's lathe gains but the breadth of a hair, but the difference…
_is_ enveloped in a larger world of _some_ sort of whose residual properties we at present can frame no positive idea. Agnostic positivism, of course, admits this principle theoretically in the most cordial terms, but insists that we must not turn it to any practical use. We have no right, this doctrine tells us, to dream dreams, or suppose anything about the unseen part of the universe, merely because to do so may be for what we are pleased to call our highest interests. We must always wait for sensible evidence for our beliefs; and where such evidence is inaccessible we must frame…