IxxiV RKPORT — 1873. 



I trust that I have now made it evident to you, that this confident expectation 

 is not justified by any absolute necessity of Nature, but arises entirely out 

 of our belief in her TJinformity ; and into the grounds of this and other 

 Primary Beliefs, Avhich serve as the foundation of all Scientific reasoning, wo 

 shall presently inquire. 



There is another class of cases, in which an equal certainty is generally 

 claimed for conclusions that seem to flow immediately from observed facts, 

 though really evolved by Intellectual processes ; the apparent simplicity and 

 directness of those processes either causing them to be entirely overlooked, 

 or veiling the assumptions on which they are based. — Thus Mr. Lockyer 

 speaks as confidently of the Sun's Chromosphere of incandescent Hydro- 

 gen, and of the local outbursts which cause it to send forth projections tens 

 of thousands of miles high, as if he had been able to capture a flask of this 

 gas, and had generated water by causing it to unite with oxygen. Yet this 

 confidence is entirely based on the assumption, that a certain line which is seen 

 in the Spectrum of a hydrogen flame, means hydrogen also when seen in the 

 spectrum of the Sun's chromosphere ; and higb as is the probability of that 

 assumption, it cannot be regarded as a demonstrated certainty, since it is by 

 no means inconceivable that the same line might bo produced by some other 

 substance at present unknown. — And so when Dr. Huggins deduces from 

 the different relative positions of certain lines in the spectra of different Stars, 

 that these Stars are moving from or towards us in space, his admirable train of 

 reasoning is based on the assumption that these lines have the same meaninr/ 

 — that is, that they represent the same elements — in every luminary. That 

 assumption, like the preceding, may be regarded as possessing a sufficiently 

 high probability to justify the reasoning based upon it ; more especially 

 since, by the other researches of that excellent observer, the same Chemical 

 elements have been detected as vapours in those filmy cloudlets which seem 

 to be stars in an eai4y stage of consolidation. But when Frankland and 

 Lockyer, seeing in the spectrum of the yellow Solar prominences a certain 

 bright line not identifiable with that of any known Terrestrial flame, attri- 

 bute this to a hypothetical new substance which they propose to call Helium, 

 it is obvious that their assumption rests on a far less secure foundation ; 

 until it shall have received that verification, which, in the case of Mr. 

 Crookes's researches on Thallium, was afforded by the actual discovery of the 

 new metal, whose presence had been indicated to him by a line in the 

 Spectrum not attributable to any substance then known. 



In a large number of other cases, moreover, our Scientific interpretations 

 are clearly matters oi judr/ment ; and this is eminently a personal act, the 

 value of its results depending in each case upon the qualifications of the 

 individual for arriving at a correct decision. The surest of such judgments 

 are those dictated by what we term " Common Sense," as to matters on 

 which there seems no room for difference of opinion, because every sane 

 person comes to the same conclusion, although he may be able to give no 

 other reason for it than that it appears to him " self-evident." Thus while 

 Philosophers have raised a thick cloud of dust in the discussion of the basis 

 of our belief in the existence of a World external to ourselves, — of the Non 

 Ego, as distinct from the Ego, — and while every Logician claims to have 

 found some flaw in the jDroof advanced by every other, — -the Common Sense of 

 Mankind has arrived at a decision that is practically worth all the arguments 

 of all the Philosophers who have fought again and again over this battle- 

 ground. And I think it can be shown that the trustworthiness of this 

 Common Sense decision arises from its dependence, not on any one set of 



