The need of an ecological quality-concep
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P. J. Schroevers
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Article
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1983
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Springer
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English
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An indicator is something which makes visible, audible or perceptible which in itself is not visible, audible or perceptible. Many times indicators concern rather concrete matters, that might be experienced also in a more direct way. Acidity can be tasted, but a pH-meter is a better instrument. If s