Pressure is often lowest at the Equator since the sun is hottest there
“Since” has a time meaning as a preposition and sometimes as a conjunction, but as a conjunction it can also give a reason like “because”.
DISPUTE ABOUT THE USES OF “Since” AND “Because”
The difference between since and because is slightly controversial among English writing experts. I have read pieces arguing that since is an acceptable alternative to because, and also that it is not. Some assert that formal “rules” do not exist regarding this question. My own belief is that rules certainly do exist: since can replace because in some cases and not others, and because can replace since in some cases and not others. In this post I wish to elaborate on that.
First it might be helpful to give my understanding of language “rules”. Within a “descriptive” approach to grammar analysis, they are essentially what people do or do not do with a word, word-part or word group. Hence they reflect common use, or frequency: if every mother-tongue speaker says something in a particular way, then it is correct; if nobody does it, it is incorrect (or at least strange); and if some do it and some do not, then it is either “variable” or controversial.
This view means that the absence of a rule from a grammar book does not mean it does not exist. Language practices have their frequencies regardless of whether or not they have been recognised and written about. As a result, rather than saying that no rule exists for separating since and because, it may be more accurate to say that few systematic objective analyses of their actual use have yet been carried out and written up – or at least that the differences have not been sufficiently or accurately described.
Most words and structures have rules of both combination (linkage with other words) and meaning (see 100. What is a Grammar Error?). To distinguish between because and since, it will be necessary to give rules of both kinds. Some are in standard reference books, but others will be based on my own extensive experience of reading and analysing academic and professional English.
COMBINATION RULES FOR “Since” AND “Because”
The combination rules are fairly simple. Both words can act as conjunctions, i.e. with a following subject + verb in a sentence with another verb (see 174. Eight Things to Know about Conjunctions), like this (verbs in capitals):
(a) Low pressure EXISTS at the Equator since/because the sun IS hottest there.
More precisely, the two words are conjunctions of the “subordinating” kind, which means they can come before the two verbs that they combine as well as between them (see 25. Conjunction Positioning).
Alternatively, since can be used as a preposition (i.e. without any following verb – see 84. Seven Things to Know About Prepositions), but its meaning is then different (see below):
(b) The true size of the earth HAS BEEN KNOWN since the voyages of Columbus.
A preposition use is also possible with because, but only if of is added:
(c) Low pressure EXISTS at the Equator because of the heat of the sun.
The meaning of this preposition use is the same as that of the conjunction one (see 72. Causal Prepositions).
MEANING RULES FOR “Since” AND “Because”
There are two meaning distinctions to be made.
1. Causal vs Non-Causal
Because is always causal; in other words, in both the conjunction and the preposition uses, it always gives a cause or reason for what the main verb says. It shows why something happens or exists (note, though, that just because… does not mean … denies a cause – see 88. Some Exotic Grammar Structures, #2).
On the other hand, since is sometimes causal and sometimes not. Sentence (a) shows a causal use, while sentence (b) shows a non-causal (“temporal”) one, meaning “from … up to now” and showing how long something happens or exists. Note how the the idea of “up to now” is included in this meaning so does not need to be made clear with separate words (see 48. Tricky Word Contrasts 1, #1).
Given that since in (a) is a conjunction and in (b) a preposition, one might think that the meaning depends on these variations. However, things are more complicated. It is true that the preposition use always expresses temporal meaning, but the conjunction use is sometimes causal, as in (a), and sometimes temporal. Here is an example of it with temporal meaning:
(d) It HAS BEEN the coldest winter since records BEGAN.
This is clearly non-causal because records began is not the reason for the coldness of the winter; we could not replace since with because. The meaning of since is definitely “from … up to now”. Notice the verb tenses that are typical with this meaning. The verb with since (began) must be past (simple or continuous), while the main verb (has been) must be present perfect (with HAVE), the normal tense for indicating “up to now” (see 171. Aspects of the Past Perfect Tense). When since is causal, both verbs can be in other tenses. Here is another example of temporal usage:
(e) Funds HAVE not BEEN WITHDRAWN since the account WAS OPENED.
Not everyone thinks the conjunction use of since can be non-causal. The reason, I suspect, is that sentences like (d) and (e), where a solely temporal meaning is clear, are not very common. In many other cases, temporal and causal meanings exist together. Consider this:
(f) Since the university LOWERED its entry requirements, many more students HAVE BEEN FAILING.
We can “feel” the presence of both meanings here: the underlined words are temporal in being the starting point of the higher failure rate and causal in being its trigger. Yet even in cases like this we can sometimes be sure that since has temporal meaning and not causal. The clue in (f) is the past tense of the since verb lowered. If the meaning were causal, this tense would have to change (in British English at least) to has lowered.
The reason why the causal meaning of since in (f) needs has lowered is that this meaning does not negate the normal rules for choosing between the present perfect and past tenses, as the temporal meaning does, and the normal rules say that a past event with no obvious time of occurrence, like last year or after the review, needs the present perfect. Note that this argument for temporal meaning could not be made if a time expression was present, since then both causal and temporal meaning would need the same past simple tense.
The meaning shown by (f) might be called “temporal with a hint of causality”, and could be worth recognising as a category alongside “purely temporal” and “purely causal”.
2. Different Kinds of Purely Causal Meaning
I have argued above that both since and because can have a purely causal meaning. However, they may still be slightly different. Consider the following examples. Both could have either because or since in the blank space, but which one is more likely to have the latter?
(g) Pressure is often low at the Equator … the sun is hottest there.
(h) The defendant used violence … he was provoked.
My subjective feeling is that (g) is more likely to have since than (h). I would suggest that this is because the outcome (low pressure) is a logical and practically inevitable consequence of the cause. The outcome in (h) is not inevitable at all, but is more specific to the person in question. This variability of outcome types is proposed in two of my other posts. It is suggested to explain the difference between the connectors as a result and therefore in the post 20. Problem Connectors, and between the conjunctions so and so that in 32. Expressing Consequences.
If this hypothesis is correct, one would expect since to be more frequent in academic and professional writing than elsewhere, since logical reasoning is so necessary and commonplace there (see 167. Ways of Arguing 1). I cannot say for sure that academic and professional writing uses since more often than other kinds do, but my extensive exposure to it gives me the strong intuition that it does.
The use of since with a logical cause in formal writing is worth comparing to the use of if with a hypothetical cause in sentences like the following:
(i) The patient is cured since the test is negative.
(j) The patient is cured if the test is negative.
Both sentences have the same evidence-like reason. However, the first states it as actually existing, while the second says only that it is a possibility (see 179. Deeper Meanings of “if”).