SECT。1 Of Knowledge
There are seven“注釋1”different kinds of philosophical relation，viz。resemblance，iden-tity，relations of time and place，proportion in quantity or number，degrees in any quali-ty，contrariety，and causation。These relations may be divided into two classes；into such as depend entirely on the ideas，which we compare together，and such as may be changed without any change in the ideas。It is from the idea of a triangle，that we dis-cover the relation of equality，which its three angles bear to two right ones；and this re-lation is invariable，as long as our idea remains the same。On the contrary，the relations of contiguity and distance betwixt two objects may be changed merely by an alteration of their place，without any change on the objects themselves or on their ideas；and the place depends on a hundred different accidents，which cannot be foreseen by the mind。It is the same case with identity and causation。Two objects，though perfectly resembling each other，and even appearing in the same place at different times，may be numerically different：And as the power，by which one object produces another，is never discover-able merely from their idea，it is evident cause and effect are relations，of which we re-ceive information from experience，and not from any abstract reasoning or reflection。There is no single phenomenon，even the most simple，which can be accounted for from the qualities of the objects，as they appear to us；or which we could foresee without the help of our memory and experience。
It appears，therefore，that of these seven philosophical relations，there remain only four，which depending solely upon ideas，can be the objects of knowledge said certain-ty。These four are resemblance，contrariety，degrees in quality，and proportions in quantity or number。Three of these relations are discoverable at first sight，and fall more properly under the province of intuition than demonstration。When any objects resemble each other，the resemblance will at first strike the eye，or rather the mind；and seldom requires a second examination。The case is the same with contrariety，and with the de-grees of any quality。We might proceed，after the same manner，in fixing the propor-tions of quantity or number，and might at one view observe a superiority or inferiority betwixt any numbers，or figures；especially where the difference is very great and re-markable。
There，remain，therefore，algebra and arithmetic as the only sciences，in which we can carry on a chain of reasoning to any degree of intricacy，and yet preserve a perfect exactness and certainty。When two numbers are so combined，as that the one has always an unite answering to every unite of the other，we pronounce them equal；and it is for want of such a standard of equality in extension，that geometry can scarce be esteemed a perfect and infallible science。
“注釋1”Part I。Sect。5.
SECT。2 Of Probability，and of the Idea of Cause and Effect
This is all I think necessary to observe concerning those four relations，which are the foundation of science；but as to the other three，which depend not upon the idea，and may be absent or present even while that remains the same，it will be proper to ex-plain them more particularly。These three relations are identity，the situations in time and place，and causation。
All kinds of reasoning consist in nothing but a comparison，and a discovery of those relations，either constant or inconstant，which two or more objects bear to each other。When both the objects are present to the senses along with the relation，we call this per-ception rather than reasoning；nor is there in this case any exercise of the thought，or any action，properly speaking，but a mere passive admission of the impressions through the organs of sensation。According to this way of thinking，we ought not to receive as reasoning any of the observations we may make concerning identity，and the relations of time and place；since in none of them the mind can go beyond what is immediately pres-ent to the senses，either to discover the real existence or the relations of objects。It is only causation，which produces such a connexion，as to give us assurance from the ex-istence or action of one object，that it was followed or preceded by any other existence or action；nor can the other two relations be ever made use of in reasoning，except so far as they either affect or are affected by it。The same reasoning extends to identity。But this conclusion beyond the impressions of our senses can be founded only on the connexion of cause and effect；nor can we otherwise have any security，that the object is not changed upon us，however much the new object may resemble that which was formerly present to the senses。
Here then it appears，that of those three relations，which depend not upon the mere ideas，the only one，that can be traced beyond our senses and informs us of existences and objects，which we do not see or feel，is causation。
The idea of causation must be derived from some relation among objects；and that relation we must now endeavour to discover。I find in the first place，that whatever ob-jects are considered as causes or effects，are contiguous；and that nothing can operate in a time or place，which is ever so little removed from those of its existence。We may therefore consider the relation of CONTIGUITY as essential to that of causation；at least may suppose it such，according to the general opinion，till we can find a more“注釋1”proper occasion to clear up this matter，by examining what objects are or are not susceptible of juxtaposition and conjunction。
The second relation I shall observe as essential to causes and effects，is not so uni-versally acknowledged，but is liable to some controversy。It is that of PRIORITY of time in the cause before the effect。Some pretend that it is not absolutely necessary a cause should precede its effect。Now if any cause may be perfectly co-temporary with its effect，it is certain，according to this maxim，that they must all of them be so；since any one of them，which retards its operation for a single moment，exerts not itself at that ver-y individual time，in which it might have operated；and therefore is no proper cause。The consequence of this would be no less than the destruction of that succession of cau-ses，which we observe in the world；and indeed，the utter annihilation of time。For if one cause were co-temporary with its effect，and this effect with its effect，and so on，it is plain there would be no such thing as succession，and all objects must be co-exist-ent。
Shall we then rest contented with these two relations of contiguity and succession，as affording a complete idea of causation？By no means。An object may be contiguous and prior to another，without being considered as its cause。There is a NECESSARY CONNEXION to be taken into consideration；and that relation is of much greater impor-tance，than any of the other two above-mentioned。
It is necessary for us to leave the direct survey of this question concerning the na-ture of that necessary connexion，which enters into our idea of cause and effect；and en-deavour to find some other questions，the examination of which will perhaps afford a hint，that may serve to clear up the present difficulty。Of these questions there occur two，which I shall proceed to examine，viz。
First，For what reason we pronounce it necessary，that every thing whose existence has a beginning，should also have a cause？
Secondly，Why we conclude，that such particular causes must necessarily have such particular effects；and what is the nature of that inference we draw from the one to the other，and of the belief we repose in it？
I shall only observe before I proceed any farther，that though the ideas of cause and effect be derived from the impressions of reflection as well as from those of sensation，yet for brevity s sake，I commonly mention only the latter as the origin of these ideas；though I desire that whatever I say of them may also extend to the former。Passions are connected with their objects and with one another；no less than external bodies are con-nected together。The same relation，then，of cause and effect，which belongs to one，must be common to all of them。
“注釋1”Part IV。Sect。5.
SECT。3 Why a Cause is Always Necessary
To begin with the first question concerning the necessity of a cause：It is a general maxim in philosophy，that whatever begins to exist，must have a cause of existence。This is commonly taken for granted in all reasonings，without any proof given or deman-ded。
All certainty arises from the comparison of ideas，and from the discovery of such re-lations as are unalterable，so long as the ideas continue the same。These relations are resemblance，proportions in quantity and number，degrees of any quality，and contrarie-ty；none of which are imply d in this proposition，Whatever has a beginning has also a cause of existence。That proposition therefore is not intuitively certain。
It is sufficient only to observe，that when we exclude all causes we really do ex-clude them，and neither suppose nothing nor the object itself to be the causes of the ex-istence；and consequently can draw no argument from the absurdity of these suppositions to prove the absurdity of that exclusion。If every thing must have a cause，it follows，that upon the exclusion of other causes we must accept of the object itself or of nothing as causes。But it is the very point in question，whether every thing must have a cause or not；and therefore，according to all just reasoning，it ought never to be taken for gran-ted。
They are still more frivolous，who say，that every effect must have a，cause，be-cause it is imply d in the very idea of effect。Every effect necessarily pre-supposes a cause；effect being a relative term，of which cause is the correlative。But this does not prove，that every being must be preceded by a cause；no more than it follows，because every husband must have a wife，that therefore every man must be marry d。The true state of the question is，whether every object，which begins to exist，must owe its exist-ence to a cause：and this I assert neither to be intuitively nor demonstratively certain，and hope to have proved it sufficiently by the foregoing arguments。
Since it is not from knowledge or any scientific reasoning，that we derive the opin-ion of the necessity of a cause to every new production，that opinion must necessarily a-rise from observation and experience。The next question，then，should naturally be，how experience gives rise to such a principle？But as I find it will be more convenient to sink this question in the following，Why we conclude，that such particular causes must necessarily have such particular effects，and why we form an inference from one to an-other？we shall make that the subject of our future enquiry。It will，perhaps，be found in the end，that the same answer will serve for both questions。