The duty of doubt and of re-examination of what tradition has bequeathed has been enforced by philosophy in all ages. For it is the cardinal principle of philosophy to be free--to possess its soul--never to be a mere machine or mere channel of tradition. But, in some ages, this assertion of its freedom has had for the soul a pre-eminently negative aspect. It has meant only freedom _from_--and not also freedom _in_ and _through_--its environing, or rather constituting, substance. Such an epoch was seen in the ancient world when the _New Academy_, with its sceptical abstention from all objective assertions, had to protest against the dogmatism of the Stoic and Epicurean schoolmen. In modern times the initial shudder before plunging in has been a recurrent crisis. Each thinker--as he personally resolved to thread his way through the wilderness of current opinion to the realm of certified truth--has had to remind himself (and his contemporaries) that in knowledge at least no possessions are secured property unless they have been earned by the sweat of their owner's brow. This is the common theme of Bacon's aphorisms in the beginning of the _Novum Organum_, of Descartes' _Discourse of Method_, and of Spinoza's unfinished essay on the _Emendation of the Intellect._ There is indeed a discrepancy in these utterances as to the measure in which they severally think it needful to insist as preliminary on a kind of moral and religious consecration of life to the service of truth. But a more compelling division arises. The maxim may be understood to say, 'Divest thy mind of its ill-gotten gains, its evil habits, prejudices, and system, and in childlike simplicity prepare thine eye and ear to receive in pure vessels the stores of truth which are ready to stream in from the world.' Or it may rather be held to say, 'Remember that thou art a conscious, waking mind, and that every idea thou hast is thine by thine own assent: insist upon thy right of free intelligence, and give no place to any belief which thou hast not raised into full light of consciousness, and found to be completely consistent with the whole power and content of thy clearest thought.' And, we may add, if the maxim be obeyed too exclusively in either way, it will be obeyed amiss.