But for some reason, that same injunction doesn't seem to apply to some academics. When you are passing authoritative judgement on GM crops, for example, it helps your credibility if you are not being funded by Monsanto, as the John Innes Research Centre discovered.
Demos is a think-tank, rather than an academic research centre, but even so there have been a few eyebrows raised about their new research about bank lending to SMEs - which was funded by Barclays of all people.
Their research found that small businesses are not actually being turned down for loans by the big banks. You might say surprise, surprise, which would be cynical. But really it doesn't matter because, even if the research was completely independent (which I'm sure it was), the fact that Barclays funded it renders it worse than useless.
I am kind of staggered by the naivety about this. A politician wouldn't get away with it. Nor would a judge. So why do scientists think being funded by an organisation absolutely committed to one particular slant will enhance their reputation for high-mindedness?
I've been thinking a little about this because of the rising clamour of the debate about the future of high streets. There are retailers who believe that out-of-town or online retailing renders the old way of shopping completely archaic. There are customers who point out that the shops moved away first, forcing people to follow. There are a whole range of points of view - these are mine.
It might seem sensible to have an academic centre dedicated to understanding the future of retailing, and the ESRC funds a unit at Southampton University to do exactly this.
But here is the bizarre thing - which renders all their publicly funded seminars and reports useless - Tesco funds it as well, and for some reason even ESRC can't see that this renders their own funding suspect.
Tesco has a very particular take on the future of high streets. I find it extraordinary that nobody at Southampton or ESRC can see that this compromises their findings, however open-minded they are being (and I am sure they are), and consequently wastes public money.
We do need an academic centre for an impartial understanding of this emerging debate. Southampton can't be it while they are accepting money from such an involved source.
And here is Tesco's UK managing director Chris Bush explaining why they are abandoning plans to build in Sherborne (it wasn't the protests, apparently). He sounds open-minded about the questions that need answering:
"The real question driving this debate is this; how do you explain why some high streets untouched by supermarkets fail while others thrive with a mix of large and small retailers trading successfully side-by-side?"
Luckily, I'm in a position to answer his question. It is about whether the supermarket is genuinely an anchor store, or whether it actually seeks to compete with all the surrounding stores - and be a health centre and post office too. The monopolist approach is corrosive, the anchor store is - or can be - genuinely, well, anchoring.
For some reason, many planning authorities don't understand the difference, because they don't measure where the money will go - whether it will stay put circulating in the local economy or whether it will be hoovered up by the non-anchor supermarket and vacuumed away elsewhere.
These are important issues. There is a great deal of research that needs doing, especially if planning authorities are going to be properly informed. But I want my data produced and interpreted by someone who is not funded by one or other of the protagonists in the argument.
So, in the meantime, next time you are presented with data about the future of high streets or anything else, don't forget to ask where it came from.