Though I have，in the second book of my Essay Concerning Human Understanding，treated of the association of ideas，yet having done it there historically，as giving a view of the understanding in this as well as its several other ways of operating，rather than designing there to enquire into the remedies [that]ought to be applied to it，it will，under this latter consideration，afford other matter of thought to those who have a mind to instruct themselves thoroughly in the right way of conducting their understandings；and that the rather because this，if I mistake not，is as frequent a cause of mistake and error in us as perhaps anything else that can be named，and is a disease of the mind as hard to be cured as any，it being a very hard thing to convince anyone that things are not so，and naturally so，as they constantly appear to him.
By this one easy and unheeded miscarriage of the understanding sandy and loose foundations become infallible principles and will not suffer themselves to be touched or questioned；such unnatural connections become by custom as natural to the mind as sun and light.Fire and warmth go together and so seem to carry with them as natural an evidence as self-evident truths themselves.And where then shall one with hopes of success begin the cure？Many men firmly embrace falsehood for truth，not only because they never thought otherwise，but also because，thus blinded as they has-e been from the beginning，they never could think otherwise，at least without a vigor of mind able to contest the empire of habit and look into its own principles?a ******* which few men have the notion of in themselves and fewer are allowed the practice of by others，it being the great art and business of the teachers and guides in most sects to suppress as much as they can this fundamental duty which every man owes himself and is the first steady step towards right and truth in the whole train of his actions and opinions.This would give one reason to suspect that such teachers are conscious to themselves of the falsehood or weakness of the tenets they profess，since they will not suffer the grounds whereon they are built to be examined；whereas those who seek truth only and desire to own and propagate nothing else freely expose their principles to the test，are pleased to have them examined，give men leave to reject them if they can；and if there be anything weak and unsound in them，are willing to have it detected，that they themselves，as well as others，may not lay any stress upon any received proposition beyond what the evidence of its truth Bill warrant and allow.
There is，I know，a great fault among all sorts of people of principling their children and scholars，which at last，when looked into，amounts to no more but ****** them imbibe their teacher's notions and tenets by an implicit faith and firmly to adhere to them Whether true or false.
What colors may be given to this or of what use it may be when practiced upon the vulgar，destined to labour and given up to the service of their bellies，I will not here enquire.But as to the ingenuous part of mankind，whose condition allows them leisure and letters and enquiry after truth，I can see no other right way of principling them but to take heed as much as may be that in their tender years ideas that have no natural cohesion come not to be united in their heads；and that this rule be often inculcated to them to be their guide in the whole course of their lives and studies，viz.，that they never suffer any ideas to be joined in their understandings in any other or stronger combination than what their own nature and correspondence give them，and that they often examine those that they find linked together in their minds，whether this association of ideas be from the visible agreement that is in the ideas themselves or from the habitual and prevailing custom of the mind joining them thus together in thinking.This is for caution against this evil，before it be thoroughly riveted by custom in the understanding；but he that would cure it when habit has established it must nicely observe the very quick and almost imperceptible motions of the mind in its habitual actions.What I have said in another place about the change of the ideas of sense into those of judgment may be proof of this.Let anyone not skilled in painting be told，when he sees bottles and tobacco pipes and other things so painted as they are in some places shown，that he does not see protuberances，and you will not convince him but by the touch；he will not believe that by an instantaneous legerdemain of his own thoughts one idea is substituted for the other.
How frequent instances may one meet with of this in the arguings of the learned，who not seldom，in two ideas that they have been accustomed to join in their minds，substitute one for the other；and，Iam apt to think，often without perceiving it themselves.This，whilst they are under the deceit of it，makes them incapable of conviction，and they applaud themselves as zealous champions for truth when indeed they are contending for error.And the confusion of two different ideas，which a customary connection of them in their minds has made to them almost one，fills their head with false views and their reasonings with false consequences.